Light Vehicle Diesel Engines, SAMPLE Chapters 4, 5, and 7 by Jones & Bartlett Learning

MASTER AUTOMOTIVE TECHNICIAN SERIES

SAMPLE CHAPTERS 4, 5, and 7

Light Vehicle Diesel Engines

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CONTENTS

Chapter 1

Strategy-Based Diagnostics

Chapter 2

Introduction to Diesel Engines

Chapter 3

Diesel Engine Emissions

Go to

Chapter 4

Cylinder Blocks and Crankshafts

Go to

Chapter 5

Cylinder Components

Chapter 6

Cylinder Heads and Valve Train Mechanisms

Chapter 7

Diesel Engine Cooling and Lubrication Systems

Chapter 8

Diesel Fuel and Low Pressure Fuel Systems

Chapter 9

Functions of High Pressure Fuel Systems

Chapter 10

Governors and Electronic Control Systems

Chapter 11

Hydraulic Nozzles

Chapter 12

HEUI Injection Systems

Chapter 13

Common Rail Fuel Systems

Chapter 14

Air Induction and Crankcase Ventilation Systems

Chapter 15

Fixed Geometry and Wastegated Turbochargers

Chapter 16

Variable Geometry and Series Turbochargers

Chapter 17

Exhaust Gas Recirculation

Chapter 18

Exhaust Aftertreatment Systems

Chapter 19

On Board Diagnostics

Go to

CHAPTER 4

Cylinder Blocks and Crankshafts Learning Objectives After reading this chapter, you will be able to: ■■

■■

4-01 Identify and explain the important factors determining diesel cylinder block design. 4-02 Identify and describe the construction and composition of diesel engine cylinder blocks. 4-03 Identify and describe the construction methods used to increase cylinder block strength and rigidity. 4-04 Identify and describe the methods used to form cylinders in an engine block.

■■

■■ ■■ ■■

4-05 Identify and explain methods used to correct cylinder wear. 4-06 Identify and explain unique construction features of diesel engine crankshafts. 4-07 Identify and explain the function of vibration dampeners. 4-08 Identify flywheel types and construction features. 4-09 Identify and explain construction features and functions of engine bearings.

You Are the Technician The construction fleet operation where you work is beginning to see a number of engines in ¾ - 1 ton service trucks wearing out after accumulating more than 500,00 miles (800,000 km) of service.You have been tasked to make service recommendations for a program to replace or rebuild all engines in-house rather than subcontract the work to an independent engine repair facility.The fleet is a mixed operation: a third of the trucks come from three major manufacturers. As part of the task, you are to develop a set of guidelines to determine the economic viability of either repairing or replacing each engine. Using the guidelines, technicians will perform inspection procedures and determine whether parts will be replaced or the entire engine exchanged for a rebuilt unit. As you set out to prepare the guidelines consider the following:

1. What type of engine blocks do you expect to encounter? Explain your answer. 2. What parts do you expect will have the greatest wear? 3. What would you expect are some of the greatest limiting factors determining the economic viability of rebuilding versus replacing the engines?

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Chapter 4 Cylinder Blocks and Crankshafts

▶▶ Introduction

Cylinder Head

Cylinder Block

The cylinder block, which encloses the cylinder bores, is the largest part and the main structure of the diesel engine. Since the diesel’s combustion characteristics and high cylinder pressure generates substantially more power output per cubic inch of displacement, it produces extraordinary vibration that requires unique design and construction features. The blocks design and other major engine components integral to the block—such as crankshafts, balance shafts, vibration dampeners, and flywheels—are examined in this chapter as well (FIGURE 4-1).

▶▶ Cylinder

Block Design

The cylinder block can be described as the largest single part and the main structure, or backbone, of the diesel engine. Its primary Crankcase function is to support cylinders and liners (if equipped), as well as major engine components such as the crankshaft and camshaft. The transmission, cylinder head(s), and other engine parts are bolted or FIGURE 4-1 The cylinder block forms the main structure of connected to the cylinder block. Diesel engine blocks have cylinders the engine. It encloses the cylinder components, and other arranged in-line, V banked, or horizontally opposed (FIGURE 4-2). major parts are attached to it. The number of cylinders used by light- and medium-duty blocks are commonly configured in four, five, six, or eight. However, many 4-01 Identify and explain the important single-cylinder, two-cylinder, and three-cylinder blocks are produced for the off-road factors determining diesel cylinder equipment market. High cylinder pressures combine with a rapid combustion phase block design. to create extraordinary vibration and torsional twisting forces in diesel blocks. What makes diesel engine blocks so distinctive are the specialized features to reduce noise, vibration, and harshness caused by these combustion forces. Many of the unique features found in diesel engine blocks are simply there to provide exceptionally stronger and more rigid block designs. These features complement, vibration-free, lightweight, durable construction also necessary for engines to perform well in automotive and other light- and medium-duty applications.

Cylinder Block Structure The diesel engine cylinder block consists of these three distinct sections: 1. the main block structure, which encloses the cylinder components 2. the cylinder component assemblies, which include the pistons, rings, wrist pins, and connecting rods 3. the crankcase, which is the area below the cylinders that encloses the crankshaft, main bearing bores, seals and crankshaft bearings.

In-Line

V-Bank

Horizontally Opposed

FIGURE 4-2 Engine block cylinder banks are configured in at least three basic ways:

in-line, V banked, and horizontally opposed.

The geartrain may be located on either the front or the rear of the engine. Geartrain rattle is a problem in diesel engines, affecting not only noise characteristic but also engine durability. More recent engine designs place the geartrain at the rear of most engines, to reduce noise and wear caused by geartrain rattle (FIGURE 4-3). Torsional vibration is the primary cause of gear rattle. Torsional vibration is produced by alternating firing impulses and compression events in the cylinders causing sharp variations to the crankshafts rotational velocity. A rhythmic speed change takes place during every combustion as the crank speeds up during poer events and slows during compression events (FIGURE 4-4). Accelerating and decelerating crankshaft results in camshaft

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Cylinder Block Design

FIGURE 4-3 Rear-mounted geartrains are quieter, with less gear wear

and more accurate valve and unit injector operation.

Intake

Compression

Bottom Dead Center

90° Before Top Dead Center

Average

Slowing

20° Before Top Dead Center

Slowest

Power

20° After Top Dead Center

Fastest

Crankshaft Speed FIGURE 4-4 Torsional vibration causes a rhythmic change in engine speed. High compression ratios and

combustion pressures create stronger torsional vibration in diesels.

and crankshaft speed variation of between 3% and 6% during each crank rotation. Changing cam and crankshaft speed causes the face of each contact tooth in the geartrain to hit the driven and back sides of opposing gear teeth face each time the engine crankshaft changes speed. The “slapping” back-and-forth action of teeth rapidly moving in and out of contact with one another produces noise and increases wear (FIGURE 4-5). Moving the

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Chapter 4 Cylinder Blocks and Crankshafts

▶▶TECHNICIAN TIP When an engine operates, the wavelike movement of accessory drive belts may casue them to appear loose. They may not in fact be loose but instead are changing in length due to torsional vibration. Torsional vibration refers to the speeding up and slowing down of the crankshaft, caused by alternating compression and power strokes of the engine cylinder. High compression ratios and even higher cylinder pressures in a diesel engine produce intense torsional vibration or rhythmic twisting forces. Torsional vibration is observed by watching the back-and-forth oscillation of belt tensioners. The change in speed of the crankshaft causes drive belts to alternately stretch and shorten, thus causing tensioner movement and belt vibration.

Gear Rattle

FIGURE 4-5 Gear rattle is caused by alternating, back-and-forth

contact between gears turning at different speeds.

geartrain to the rear of the engine, where torsional vibrations are reduced due to the heavier flywheel mass, can dramatically reduce geartrain noise. Gear life and valve timing accuracy is improved.

▶▶ Cylinder 4-02 Identify and describe the construction and composition of diesel engine cylinder blocks.

Block Materials

Most diesel cylinder blocks today are manufactured from cast iron but aluminum and a newer block material made from compacted graphite iron (CGI) are now popular in many light-duty engines. Ford’s 6.7 L Powerstroke and the Lion V6 for the Ford F-150 use a CGI block with aluminum cylinder heads to create a stronger but lighter block than is possible with ordinary cast iron (FIGURE 4-6). Various European diesels from Mercedes and Volkswagen (VW) also use CGI blocks.

Cast Iron Blocks In contrast to CGI, ordinary cast iron is a metal made from iron ore with a carbon content of between 3% and 5%. The carbon is scattered throughout the metal in flakes, creating metal with a crystalline-like structure. The metal is poured or cast into molds commonly made from sand or fiberglass. The main advantages of cast iron are that it absorbs vibration well and that it is easy to manufacture and machine, inherently strong, and corrosion-resistant and heat-resistant. The high carbon graphite content of iron makes it an ideal wear surface for moving parts such as pistons. To improve strength, hardness, and corrosion resistance, manufacturers may alloy the iron by adding nickel, molybdenum, and chromium. Cast iron is still not as strong as steel, so bolt threads are usually cut with a deeper coarse thread pitch to minimize the problem of stripping threads. Main bearing caps are often hardened, made from steel, or use special alloys to provide improved block strength.

Casting and Seasoning Cast Iron Blocks

FIGURE 4-6 The Ford 6.7 L Powerstroke uses a CGI

cylinder block with aluminum cylinder heads.

Sand casting blocks made from sand molds require the removal of sand from inner passageways (FIGURE 4-7). Core plugs, sometimes called frost plugs, are used to cover holes made in the block to flush sand out of small passageways. Coolant heaters are often installed in the core plugs, but the core plug is not used to prevent damage to a block caused by frozen coolant. The plugs are mistakenly called frost plugs since they are pushed out by frozen coolant. Engine block damage is still often the result even after frozen coolant pushes out a core plug. After rough casting, blocks tend to warp. To minimize distortion, diesel blocks are often stored at high temperatures for a day or two to heat-treat, or as it is occasionally described “season,” them. Prolonged heating tends to relieve stress from the block and reduce any future distortion. Metallurgical changes to make the iron less brittle also take place while heat-treating the block. Subsequent to seasoning, castings are bored, tapped, and machined to finish. To

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Cylinder Block Materials

FIGURE 4-7 Core plugs are installed in a block only after casting

FIGURE 4-8 The arrow shows an area of discoloration where the

sand, which is used to form the blocks internal passageways, is cleaned out of the block passage through pressure washing.

upper third of the 6.6 L GM Duramax cylinder has been induction hardened in the ring turnaround area, where there is the least amount of cylinder lubrication.

increase the hardness and durability of cast iron cylinder blocks, they may be induction hardened. Induction hardening is a heat treatment process that involves passing alternating electric current through coils of heavy-gauge wire surrounding the material to be hardened. Through the principle of magnetic induction, intense localized heating takes place on the surface of the metal, which is then quenched with water to produce a hard, wear-resistant metal surface. GM’s Duramax 6.6 L blocks induction harden the upper third of the cylinder walls. Induction hardening increases engine life by minimizing cylinder wall wear in the ring turnaround area, where there is the least amount of cylinder lubrication (FIGURE 4-8).

Disadvantages of Cast Iron Cast iron is heavy, and engines made from cast iron add considerable weight to the vehicle in comparison to lighter materials such as aluminum or CGI. Heavy engines can negatively affect vehicle handling and braking characteristics. These heavier vehicles tend to dive more when braking, steer less nimbly, and require heavier front suspension systems. Cast iron does not transfer heat as readily as aluminum. Cast iron cannot be easily repaired by welding, like steel or other metals can. Most often, broken or cracked parts require replacement. However, some block porosity problems, such as those causing external oil leaks, can be repaired with special epoxies. Original equipment manufacturer (OEM) repair recommendation for external block porosity is to use epoxy in areas where mechanical stress is not significant. In high-value blocks, welding techniques to repair cracks involve preheating the block and using a welding rod with high nickel content or a powdered iron welding rod.

Aluminum Aluminum is a block material that is being used more commonly for high-performance diesels. Advances in manufacturing and hardening techniques have enable this lighter material to be used in high-performance applications. In comparison to cast iron, aluminum in small, six-cylinder diesel engines can eliminate more than 77 lb (35 kg) of weight. But until the late 1990s, aluminum engine blocks were used only in gasoline-fueled engines because high-performance diesel engines are required to withstand higher combustion temperatures and three times more cylinder pressure than spark-ignition, gasoline-fueled engines. Aluminum also has challenges in diesel applications, in terms of rigidity, strength, and reliably bolting the cylinder head and main bearings. Although it transmits heat well,

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Chapter 4 Cylinder Blocks and Crankshafts

FIGURE 4-9 This Mercedes 3.0 L Bluetech engine is

a die cast aluminum block with a “cast-in-place” iron cylinder sleeve.

aluminum has an expansion coefficient of four times the rate of iron. This can cause critical dimensions such as the crankshaft bearing bores and cylinder bore clearances to enlarge. Installing head and main bearing cap bolts with enough clamping force is another problem. With aluminum, there is greater potential to strip bolt threads if fasteners are tightened too much or cylinder pressures forcing the crankshaft downwards through the oil pan. To overcome this problem, a technique similar to seasoning cast iron blocks, precipitation hardening, also called particle hardening, is a widely used heat treatment technique used to increase the strength of aluminum. By alloying the aluminum with a small percentage of unique, proprietary particles, combined with prolonged heat-treating and slow cooling, aluminum blocks can achieve additional strength and hardness. While aluminum can be hardened, the greatest challenge with aluminum is that it cannot withstand the friction between pistons and the cylinder wall without rapidly wearing. However, because of aluminum’s weight-saving advantage, other manufacturing techniques have been employed to overcome these difficulties. For example, in aluminum blocks, cylinder walls and bearing bores are made from using casting iron liners to provide durable, low-friction wear surfaces (FIGURE 4-9). Another technique is to apply another very hard material to cylinder wall surfaces. Spray welding, called plasma welding, applies an alloyed iron coating over the cylinder wall. Tool steel hardness coatings of iron carbon alloys can be applied by plasma spraying the cylinder walls using a rotating plasma torch and metal alloy powder. The coating which is a few tenths of a millimeter thick to give the cylinder walls great wear resistance and durability.

Nanoslide Technology To reduce friction and increase wear resistance, Mercedes-Benz developed a spray welding technique to coat cylinder walls of the 3.0 L block with a tool-grade hardness iron alloy (FIGURE 4-10). The technique, known as Nanoslide technology, first melts wires of iron/ carbon alloy in an electric arc. Pressurized gas deposits only a 0.1–0.15 mm thick lining of the melted material onto the cylinder walls (FIGURE 4-11). This ultra-fine nano-crystalline coating is then finished to a smooth mirrorlike surface by using a special laser honing process that melts the wall and permits nitrogen gas to blend into the coating. In spite of the extreme smoothness, the nitride hardening of the cylinder wall during honing opens pores in the material that are able to retain oil for lubricating the piston and cylinder wall. The result is an ultra-low-friction surface where mechanical losses due to friction are reduced by up to 50% in comparison to gray cast iron cylinder liners. In addition, extremely high wear resistance is imparted to the cylinder walls. Other advantages include lower engine weight, reduced fuel consumption, and lower emissions. Wire Feed

Spray Wire Contact Wiring Cylinder Wall Coating Spray

Power + Supply – Nanocrystalline Iron Coating

Gas Supply Arc Atomizing Gas

FIGURE 4-10 Nanoslide technology involves spraying a thin coating or extremely hard iron alloy onto the cylinder walls of an

aluminum engine. T he coating is further hardened through nitriding while laser honing of the cylinder wall.

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Cylinder Block Materials

Conventional Design

Cylinder Head Bolt

Mirror Bore Coating

Cast Iron Liner (0.08" [2 mm]) Mirror Bore Coating (0.008" [0.2 mm])

Aluminum Cylinder Head

Aluminum Cylinder Piston

FIGURE 4-11 The cylinder walls

Tie Bolt

Block

of a Mercedes 3.0 L engine have a thin layer of hardened iron alloy to prevent wear on the aluminum cylinder block walls.

Water Jacket

Cast-in-Place Cylinder Sleeves Lightweight aluminum engine blocks can use hardened sleeves to withstand the friction from reciprocating pistons and rings. Aluminum blocks with hardened iron sleeves are constructed with a cast-in-place sleeve, where the aluminum block is molded around a cylinder sleeve that is placed into the mold before molten aluminum is poured to form the cylinder block. GM’s 1.6 L Chevy whisper diesel uses an aluminum block construction with a cast iron sleeve. A cast iron plate bolted to the bottom of the crankcase further stiffens the block.

Aluminum Block Design Variations Since aluminum blocks are softer metal, one problem is the potential of stripping bolt threads. Small aluminum blocks cannot withstand the tensional loading on bolt threads without stripping threads out of the block. To overcome this problem, blocks can be made with steel thread inserts. One technique is to tie the cylinder head to the block with steel tie bolts inserted through the bottom of the block. This construction technique, called the warp anchor method, clamps the cylinder head to the block by using head bolts threaded into steel sleeves locked into the block (FIGURE 4-12). The block accepts the cylinder head bolt from one side and the tie bolt from the other. Another technique, called the tension bolt design, is almost identical to the warp anchor bolt construction, except that it anchors the bolt through the main bearing cap or main bearing girdle, which is a steel plate attached to the bottom of the crankcase on ladder block designs (FIGURE 4-13).

FIGURE 4-12 Aluminum cylinder

blocks can use a warp anchor principle to prevent distortion of the block from cylinder head bolts. This means the cylinder head and the cylinder block are bolted together by tie bolts. Sliding steel sleeves, which are locked in the block, accept the cylinder head bolt from one side and the tie bolt from the other, preventing the block from being distorted.

Cylinder Head Bolt

Aluminum Cylinder Head

Open Versus Closed Block Decks Aluminum engine blocks are commonly die cast. This involves squeezing molten aluminum between two steel halves of a stamping die to produce a finished part. The advantages include improved block dimensional accuracy, smother surfaces, the ability to use thinner block walls, Aluminum and eliminating secondary machining in comparison to cast iron or aluminum blocks. In fact, Cylinder die casting is a faster production method and a less expensive method than casting blocks Block Tie Bolt using sand or fiberglass molds. Sleeves and steel inserts can also be cast in place. Honda, Mercedes, and GM are examples of OEMs that use die cast blocks. Honda uses a proprietary ASCT (Advanced Semisolid Casting Technology) for it’s die cast aluminum 2.2L 4-cylinder i-CTDi (Intelligent Common rail Turbo Diesel Injection) engine. Aluminum used for the casting has a “mushy” consistency when it is injected into dies and pressurized before the dies are separated. Using this method, cylinders can be separated with aluminum that is as thin as 8 mm (0.315") to form a very compact engine that is 30% lighter than cast iron. These blocks FIGURE 4-13 Tension bolt blocks anchors are specially heat-treated and particle hardened, followed by a plasma sprayed coating of hard the cylinder head bolts through the main iron alloy applied to the cylinder walls. Cast iron main bearing caps are used in this engine bearing cap or main bearing girdle on that delivers a combined fuel economy of 40.5 US-mpg (5.8 L/100 km) in the Accord wagon. ladder block designs.

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Chapter 4 Cylinder Blocks and Crankshafts

FIGURE 4-14 This die cast block

uses an open-block deck. There is no support at the top of the cylinder that bridges the cylinder walls with the rest of the block.

In contrast to cast iron blocks, die cast blocks have what is known as an open-deck design (FIGURE 4-14). Open decks are easily identified since the block deck—the top surface of the block, in contact with the cylinder head—has no connecting metal between the block walls and the upper portion of the cylinder wall. Although coolant can better circulate in open deck blocks, the design has the disadvantage of an absence of reinforcement in the upper cylinder walls. Without a metal bridge between the cylinder walls and block deck the weakly supported cylinder walls are capable of moving during service. Closed decks connect the block deck with the cylinder walls to form a stronger, more rigid cylinder structure (FIGURE 4-15).

Compacted Graphite Iron Using CGI, or sintered graphite, for a block material is the latest engine block construction technique. CGI engine blocks provide blocks with greater strength, rigidity, and durability at a reduced weight and brittleness compared to cast iron. Theoretically, a CGI block can be constructed with the strength of steel but with almost the weight of aluminum. The stronger, steel-like-strength material enables the use of thinner block and cylinder walls without sacrificing stability or strength. CGI starts with powdered iron and graphite alloys squeezed into molds at high pressures and then heated to bond the metal particles together. This construction technique yields blocks with at least 80% higher tensile strength, 45% greater stiffness, and approximately double the fatigue strength of conventional gray cast iron and aluminum. The weight of an engine block made from CGI alone can be reduced by 22% through a 15% reduction in the FIGURE 4-15 Closed blocks connect the top of the cylinder wall with the block deck through the use of a metal bridge. This thickness of the block walls. Noise transmission through the block is bridge closes the gap between the cylinder and the rest of the dramatically reduced as well, because of the vermicular (wormlike) block. grain structure of the material, which has sound-absorbing properties. By reducing the block size and weight while increasing strength, CGI materials allow engine designers to improve power density, performance, fuel economy, and durability while reducing engine weight and noise. Many light-, medium-, and heavy-duty diesels are now made from CGI, including blocks from Audi, Cummins, VM Motori (Dodge 3.0 L Eco diesel), and Fords F-150 3.0 L introduced in 2018 and 6.7 L Powerstroke introduced in 2010. Ford claims that the 6.7 L Powerstroke is a made from patented SinterCast CGI material that is 40% more rigid and 100% more fatigue-resistant than gray cast iron (FIGURE 4-16). CGI has allowed Ford designers to reduce the engine wall thickness by 15% in comparison to the thickness that would be needed if it used gray cast iron. A weight reduction of 160 lb (70 kg) is achieved in comparison to the earlier cast iron 6.4 L engine.

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Block Crankcase Construction

FIGURE 4-16 The EcoDiesel 3.0 L

from Fiat Chrysler Automobiles (FCA) used in the Dodge Ram uses a CGI engine block.

CGI engine blocks have their own unique service challenges in terms of machining. CGI is steel-like in hardness but also abrasive and “gummy” when machined. Special machine tools with extraordinary harness are needed to cut the material. Honing cylinders are much harder, and a coarse grit honing stone will produce a smoother finish than the same stone will produce in a cast iron block. These blocks cannot be repaired when damaged, so they must be replaced.

▶▶ Block

Crankcase Construction

Blocks must be designed to be as compact as possible to reduce weight and size while maintaining strength and rigidity. Unnecessary weight is the enemy of good vehicle dynamics, such as braking, precise handling, and quick acceleration. However, engine weight translates to strength and rigidity, two important features of an engine to reduce vibration, noise, and harshness (FIGURE 4-17). Three basic types of block construction are used to best turn weight into strength. Construction techniques used by these blocks reinforce the crankcase part of the

4-03 Identify and describe the construction methods used to increase cylinder block strength and rigidity.

Oil Pan

Main Bearing Caps

Main Bearing Caps Directional Marking Crankshaft Bore Camshaft Bore and Bushing

FIGURE 4-17 A low power output cast iron block design. The block walls extend only to the main

bearing caps to reduce the height of the block. Only the oil pan forms the crankcase and reinforces the block strength.

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Chapter 4 Cylinder Blocks and Crankshafts

cylinder block, which in turn will increase rigidity to resist bending and noise generating vibration as the power strokes on the crankshaft transmits energy capable of distorting bearing bores.

Deep-Skirt Blocks

FIGURE 4-18 Deep-skirt blocks extend the block walls

below the main bearing caps. Note also the direction indicators for the road bearing caps point toward the front of the engine.

Anchor Bolts to Main Bearing Cap Oil Pan Rails

Reinforcing Steel Bedplate

A deep-skirt block is a design that refers to a block configuration where the bottom edge of the block or crankcase walls extends well below the crankshaft’s centerline (FIGURE 4-18). The sides of the crankcase are separated from the main bearing caps to minimize transmitting the noise produced by combustion events out from the main bearing caps. Bearing caps are held in place with two or four vertically placed bolts. Lengthening the block walls below the main bearing caps enhances structural rigidity and operating smoothness which allows for a larger mating surface with the transmission. A reinforcing steel plate may also be attached to the oil pan rails and main bearing caps to enhance rigidity and strength of the block. The plate, whose thickness is approximately ⅜"–½" of steel, ties the oil pan rails to the main bearing caps to add further structural rigidity to the block (FIGURE 4-19). Reducing vibration and increasing block rigidity can be further enhanced by adding curves to the deep-skirt crankcase walls. Also known as serpentine convolutions, the curves make the crankcase less flexible than flat surfaces would be. Adding ribs and webbing to the crankcase area is a design technique that minimizes crankcase distortions that would cause noise vibration and harshness in all types of blocks (FIGURE 4-20).

Crossbolted Blocks FIGURE 4-19 A steel bedplate attached to the oil pan rails and

the main bearing caps of this high-torque-output, deep-skirt engine block adds extra rigidity and strength to the block.

Advancing beyond the traditional deep-skirt block technique is the crossbolted block. Also known as tie-bolted or bolster-bolted blocks, this design uses additional horizontal placed bolts to connect crankcase walls of the block to the main bearing caps (FIGURE 4-21). Crossbolted

Closed Block Deck

Crossbolted Block FIGURE 4-21 A crossbolted main bearing cap in an aluminum

FIGURE 4-20 The crankcase of this 5.0 L Cummins diesel used

by Nissan pickups has a serpentine wall. The design adds rigidity and strength to its deep-skirt design.

block with cast iron liners. Horizontal crossbolts or tie bolts attach the main bearing caps to the crankcase wall. A pair of vertical bolts also ties the main bearing cap to the block. Crossbolting is used to enhance engine block strength and rigidity while reducing noise and lowering engine weight.

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Block Crankcase Construction

blocks permit the use of more than the traditional two or four vertical bolts to hold the main bearing caps. Main bearing caps fit tightly between the short walls of the crankcase and are bolted both vertically and horizontally. Crossbolting adds the structural strength of crankcase walls to resist crankshaft flexing, and it stiffens the engine’s structure, to reduce overall vibration. This type of block reinforcement technique permits further weight reductions because the crankcase walls can be shorter than in a deep-skirt block. Block height can also be lowered.

Ladder-Frame Blocks A number of newer engine designs are incorporating ladder-frame construction features, which can strengthen a block while reducing weight, noise, and vibration (FIGURE 4-22). In ladder-frame blocks, the sides of the block extend exactly to the centerline of the crankshaft bearing bore. A separate additional section of the block structure, called a ladder frame, attaches to the crankcase and the oil pan (FIGURE 4-23). The main bearing caps are all integrated in this lower single-piece section of the block. Removing one bearing cap requires removing the entire ladder frame, which contains all the bearing caps. It is called a ladder-frame construction due to the ladder-like appearance of the crankcase-mounted bearing girdle (FIGURE 4-24). The ladder frame may also contain balance shaft housings and the oil pump. Although ladder frames add considerably to block rigidity and strength, they also can add to the cost and complexity of an engine. Another drawback, depending on construction technique, is that the gasket joint created by the device has the potential for leakage. Engine bearings cannot be serviced in frame or in chassis. Removing and replacing a bearing requires completely removing of the engine from the vehicle.

Tunnel-Bore Blocks

FIGURE 4-22 The 6.0 L Powerstroke and 6.4 L Powerstroke

Tunnel-bore blocks are one of the most smoothly operating and strongest of any cylinder block design. Integrating the main bearing bores into a single-piece block structure provides the highest strength and rigidity to the block. In a tunnel-bore block, as the name suggests, the crankshaft and main bearing enclosures are surrounded by the “tunnel” cast into the block (FIGURE 4-25). The crankshaft bearings are held in place by bearing support structures. After the bearing support structures are bolted around the crankshaft journals, the entire crankshaft assembly is lowered into the tunnel bore of the One-Piece Block Bolts

Main Bearing Cap Bolts

have a ladder-frame engine block. A ladder frame uses an additional block structure located between the cylinder block and oil pan, which incorporates the main bearing caps into a one piece component resembling a ladder, sometimes called a girdle.

Bell Housing

Connecting Roads

Ladder Frame-to-Block Bolts FIGURE 4-23 This 6.0 L ladder-frame block uses a two-piece

crankcase. The upper section incorporates the main bearing caps into the crankcase structure.

FIGURE 4-24 The ladder-frame block incorporates the main

bearing caps into a single structural component, resembling the rungs of a ladder. The frame not only supports the crankshaft but also adds more structural rigidity and strength to the block, producing substantially less noise and vibration.

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Chapter 4 Cylinder Blocks and Crankshafts

Holes for Main Bearing Support Locating Bolts

Main Bearing Support

FIGURE 4-25 In this tunnel-bore engine

Crankshaft Tunnel

Crankshaft Main Bearing Support

block, the crankshaft and its main bearing supports are dropped into the tunnel during assembly. Locating bolts in the block aligns oil supply holes and prevents the main bearing supports from rotating with the crank.

block. Locating bolts in the block aligns oil passageways in the bearing supports structures with passages in the block. The locating bolts also lock the main bearing supports into position. No automobile use these blocks, but Kubota supplies these blocks for use in compact off-road equipment.

▶▶ Engine 4-04 Identify and describe the methods used to form cylinders in an engine block.

Block Cylinder Configurations

The cylinder support section of the engine block uses several configurations, depending on the application, production cost, and expected vehicle lifecycle. Common cylinder construction types include the parent bore, wet sleeve, and dry sleeve, as well as the castin-place sleeve previously mentioned in the section aluminum blocks.

Parent Bore Block A parent bore block, also commonly called a no-sleeve block, has holes cast and bored in the block for the cylinders. The pistons are inserted directly into these cylinder holes. No provision is made to add a cylinder sleeve to this type of block. The cylinder walls form a part of the block structure, giving it rigidity and strength. Parent bore blocks can be either the open-deck or the closed-deck type, with open-deck blocks fabricated by using forging dies rather than constructed through casting techniques (FIGURE 4-26).

Advantages The major advantage of parent bore blocks are the lower initial cost of construction because machining and fitting cylinder sleeves is not required. Unlike sleeved blocks, the cylinder walls are structural members of the casting and add strength and rigidity to the block while

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Engine Block Cylinder Configurations

FIGURE 4-26 The 6.7 L

Cummins on the right is an example of a parent bore block. A parent bore block forms the cylinders and block from a single casting. Water jackets and other internal passageways are cast into the block.

using less block mass. Thin cylinder walls with minimal material thickness between cylinders permit compact, lightweight construction. These blocks are best used in applications where chassis life and expected engine life are approximately equal. Water jackets formed inside the block permit cooling of cylinder walls all the way to the top of the block, unlike sleeved engines. This feature reduces carbon formation on pistons and other problems related to excess heat near the top of the cylinder.

Disadvantages

▶▶TECHNICIAN TIP

A major disadvantage of parent bore blocks is that while rebuilding or repairing the engine, a worn cylinder must be re-bored and honed. Re-boring requires special equipment available only at a machine shop. The engine must usually be removed from the chassis and disassembled completely to perform the machining process. The number of times the engine can be rebuilt is limited to available piston oversizes and the thickness of the cylinder walls. Parent bore blocks have reduced engine block life as compared to sleeved engines. Part of the reason is the block materials form the cylinder walls. Without costly alloys and hardening techniques, the cast iron cylinder walls will wear at a faster rate than hardened cylinder sleeves will. However, parent bore CGI blocks do not wear as fast as cast iron blocks. Since conventional cast cylinder walls are irregularly thick, heat transfer differentials cause varying cylinder dimensions and accelerated wear. Cast iron cylinder wall surfaces are not as easily hardened as sleeved engines.

Coolant temperatures are hotter at the rear of the engine, where coolant has to travel the farthest to remove heat. A 10–20°F temperature differential exists between the temperature of cooler coolant near the front of the block and the rear of the block, which is farthest from the radiator. Some engine rebuilders recommend a larger piston-to-cylinder wall clearance in the rearmost cylinders of some V8 diesel engines, to accommodate the greater thermal expansion of parts.

Cylinder Sleeves Sleeves—or liners, as they are commonly called—are replaceable wear surfaces making up the cylinder wall (FIGURE 4-27). Sleeved engines are not used in light duty diesels due to their high cost, but the large numbers of in service sleeved engines justifies a brief discussion in a survey of diesel block construction techniques. Constructed to the exact inside diameters and lengths, the cylinder sleeves may be replaced individually if they become worn or damaged prematurely. Cylinder sleeves are typically induction hardened and manufactured using special alloys to produce durable, wear-resistant surfaces. Sleeves are commonly used

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Chapter 4 Cylinder Blocks and Crankshafts

in medium and large bore engines operating under high-mileage, extended service conditions. Today’s heavy-duty diesel truck with sleeved cylinders will operate for up to 1 million miles or close to 1.6 million km before needing overhaul. Long distance travel means that an engine can wear out faster than the chassis. To avoid expensive engine or vehicle replacement, replacing the liner sleeves by using cylinder kits made up of pistons, rings, and liners restores engine to factory specifications.

Dry Sleeve Block

FIGURE 4-27 Replaceable cylinder sleeves enable a worn-out

engine to be quickly rebuilt and restored to factory specifications.

A dry sleeve block is designed with a bored or honed hole in the block that allows no coolant contact with the cylinder sleeve. The sleeve is inserted into the bored hole as either a slip or press fit. The liner is installed with an interference fit that will not usually have a flange and is machined flush with the block deck. A slip fit liner will use a counter bore machined into the block to support the bottom of the liner. Pressure applied by the cylinder head to a slight protrusion of liner flange will prevent liner movement.

Advantages A replaceable cylinder sleeve can be used during block manufacturing to restore the cylinder block to its original specification after worn sleeves are removed and new ones installed. Since the sleeve is fitted into a bored hole in the block, it does not have contact with coolant that could cause sleeve deterioration. Additionally, the absence of coolant contact means there is no need to seal the sleeve to prevent coolant leakage into the crankcase. Cylinder sleeves may be replaced individually if they become worn prematurely or damaged. Disadvantages Since the coolant is not in direct contact with the dry sleeve, heat transfer from the combustion chamber to the coolant water is not as rapid as it would be with a wet-type sleeve. This slow heat transfer may potentially result in short engine life, cylinder damage, and limitations to power output per cylinder. Extra steps in manufacturing and machining the block are required for sleeve installation. Dry sleeved blocks tend to weigh more than parent bore blocks.

Wet Sleeve Block A wet sleeve block is designed with a number of large holes into which the cylinder sleeves are inserted. Coolant has direct contact with the outside of the sleeve, and there is no supporting cylinder bore structure, like a dry sleeve around the wet sleeve. Wet sleeves are thick enough to withstand the higher heat loads and forces of combustion in high horsepower engines (FIGURE 4-28). Advantages The major advantage of wet sleeve liners is the direct contact of coolant with the sleeve that enables rapid heat transfer from the cylinder to the coolant. Sleeves are easily removed and installed during engine rebuilding to restore the cylinders to original specifications.

FIGURE 4-28 Wet liners make direct contact with coolant. O-rings

prevent coolant from leaking out of the water jackets and into the oil pan.

Disadvantages The major disadvantages with wet sleeve liners are the problems associated with preventing internal engine coolant leaks. The O-ring seal (if used) at the lower part of the liner can deteriorate and leak coolant into the lubrication oil. Additionally, liner vibration caused by combustion forces can exaggerate the effects of cavitation. Greater attention to maintaining the

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Cylinder Block Service

▶▶TECHNICIAN TIP

FIGURE 4-29 Pin holes in this liner are caused by cavitation erosion.

Cavitation erosion produces pinholes in cylinder block walls, injector tubes, water pumps, and cylinder heads.

cooling system is required in wet sleeve engines, to prevent liner cavitation. Cavitation erosion is a condition caused by liner vibration, producing pinhole size holes in the sleeve (FIGURE 4-29). The holes enable coolant to enter the cylinder or combustion gases to enter and pressurize the cooling system. A second problem is ensuring that liner protrusion above the block deck is within specification and even across all cylinders. Without correct protrusion, combustion gases will leak into the coolant; head gaskets will leak; and other structure problems can result. Further information about various types of liners and liner service are covered in the textbook Fundamentals of Medium/Heavy Duty Diesel Engines.

▶▶ Cylinder

Parent bore and sleeved diesel engines have a unique problem, known as cavitation erosion. Cavitation erosion is caused by the collapse of tiny water vapor bubbles formed when coolant vaporizes on hot cylinder wall surfaces. These vapor bubbles collapse when cooled by surrounding coolant or when pressure increases around the vapor bubble. Pressure exerted by the force of bubble collapse is reported to be as high as 60,000 psi (413,700 kPa). The force of vapor bubble implosion against cylinder walls, cylinder liners, and other components can eventually perforate them. For this reason, diesel engines require the addition of supplemental coolant additives to the cooling system, which contains nitrite. The nitrite chemical additive forms a sacrificial barrier to protect the coolant side of the cylinder wall or liner against pitting due to cavitation erosion, which can quickly destroy the cylinder walls and liners.

Block Service

Cylinder block service primarily deals with wear caused by high engine hours, accumulated distances, and contaminants. Diesel engines can outlast spark-ignition counterparts for several reasons. The primary reason is slower engine speeds. Maximum engine speed is lower and torque output can reach its peak at engine speeds just above idle. This eliminates the need for additional gearing to boost torque while keeping engine rpm low. A second reason is that gasoline-fueled engines tend to wash friction reducing lubrication oil from cylinder walls. Since fuel does not enter the cylinders of a diesel during intake stroke, washing and diluting cylinder wall lubricant does not take place, which enables greater engine longevity. More durable cylinder and block materials can withstand higher combustion forces, which helps to further extend engine life. Small bore diesels can last

4-05 Identify and explain methods used to correct cylinder wear.

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Chapter 4 Cylinder Blocks and Crankshafts Cylinder Ridge Wear Step Greatest Wear

three to four times longer than spark-ignition engines, but eventually, cylinders wear and require service (FIGURE 4-30). Dirt ingestion through leaking air intakes or coolant contamination of engine oil can take place, which also dramatically accelerates engine wear. For this reason, cylinder block service consists primarily of repairing and restoring the cylinder wall finish.

Cylinder Wall Finish

Ring Wear

No Ring Travel Least Wear

FIGURE 4-30 Wear patterns in a cylinder. A carbon

ridge is formed from carbon deposits on the untraveled portion of the cylinder wall surface above the top ring. The ridge must be removed prior to piston removal to prevent damage to the piston and rings as they pass through the ridge.

Cylinder walls have a crosshatch finish made by honing stones, which serves two purposes. The first is to retain oil. Lubricating oil is left in the fine lines of the crosshatch to help lubricate rings and pistons. The fine lines will intersect one another at an angle of between 45° and 60° and are deposited with lubricating oil by the pistons oil control ring (FIGURE 4-31). A second purpose for a crosshatch finish is to provide a surface to enhance sealing piston rings and the cylinder wall. During the initial engine break-in period, the finer lines are more easily worn away than a solid wall of iron. Because a cylinder is not perfectly symmetrical, the crosshatch will allow initial wear of the ring face material into the cylinder wall to compensate for irregular ring – wall contact, which eventually produces a tighter seal between the two parts. Highly polished cylinder walls having no crosshatch will not allow residual oil to cling to the cylinder walls. Without the critical oil film on the cylinder walls, a gas-tight piston ring seal cannot be achieved. Consequently, poor sealing between the piston ring and cylinder wall causes compression and power loss due to blowby and piston and ring scuffing. Poor ring sealing also leads to high oil consumption because oil is not scraped from the cylinder wall and can flood the combustion chamber (FIGURE 4-32). Cylinder crosshatch is installed when an engine is manufactured and during proper cylinder reconditioning processes during an engine rebuild. Honing stones are used to produce the crosshatch finish of the cylinder walls after they are bored to a nominal size. To reduce the time and distance required for engine break-in, manufacturers use a process called plateau honing. This involves honing the cylinders and then removing the peaks of the cylinder crosshatch with specialized brushes generally made of nylon-like material. Plateau honing also reduces engine oil consumption during break-in.

Bottom Major Thrust Wear

Cross-Hatch

Wear-Little Cross-Hatch

Ring Turnaround

Top

FIGURE 4-32 A missing crosshatch is evidence of excessive cylinder wear. FIGURE 4-31 Crosshatch cylinder finish lines intersect each

other and retain lubricating oil.

Having a crosshatch on the cylinder wall is important to provide a surface for piston rings to wear into, or seat, during the break-in period.

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Cylinder Block Service

Cylinder Wall Wear Diesel engine cylinder walls last longer than their gasoline counterparts, for several reasons. Slower engine speeds reduce the amount of piston travel. Diesel fuel can also act like a lubricant to minimize wear from friction—unlike gasoline, which is a solvent for lubricating oil. Special alloys and processes that harden cylinder walls further increase the durability of diesel blocks. However, cylinder block walls progressively wear with increasing engine hours and distance traveled. The greatest amount of wear occurs near the uppermost part of the compression ring travel and diminishes downwards over the ring swept area of the cylinder. This gives a worn cylinder a slight bell-shaped wear pattern, with the widest dimension observed at the top of the cylinder. The reasons for the greatest amount of wear near the top of the cylinder wall causing the cylinder wall to taper include the following: ■■

■■

Combustion Pressure

Piston

Side Thrust

Higher cylinder pressures near the top part of piston travel increase ring pressure against the cylinder walls. (Gas pressure behind the rings forces the piston rings outward from their grooves.) Higher cylinder temperatures make the cylinder wall softer and more prone to wear. The lack of upper cylinder wall lubricant is due to the fact that lubrication oil can be carried into this area only on the face of the compression ring. The oil control ring is lower on piston and below the compression ring.

Piston Pin Major Thrust Face Minor Thrust Face Connecting Rod

Crankshaft Rotation

Long stoke engines will have more wear mid-stoke as well, due FIGURE 4-33 The angle formed between the connecting rod to greater forces of major thrust. The side thrust is produced by the and the piston contributes to side thrust wear of the cylinder angle of the connecting rod with the piston, which tends to push the wall. piston sideways into the cylinder wall when cylinder pressures are high (FIGURE 4-33). The angle formed between the connecting rod and the piston forces the piston against the cylinder wall during compression and power stroke. During power stroke, the force is greatest, producing a major thrust force and a minor thrust force during compression stroke. The longer stroke distance of the diesel makes for higher thrust angle forces.

Dusted Cylinders Diesel engines are air-breathing machines that consume tremendous quantities of air due to their lean burn combustion principles. Any leak in the air intake system can allow dust and dirt into the cylinders, where it can stick to the oil film on the cylinder walls. Lubricating oil provides a sticky surface to allow the dirt to adhere to the cylinder wall, which forms an abrasive lapping compound for the cylinders and pistons rings. When engine wear is rapidly accelerated due to air intake leaks, the condition is referred to as “dusting out.” Coolant may accelerate wear too, but it more rapidly destroys engine bearings before contributing to significant cylinder wall wear.

Measuring Cylinder Wall Wear Several methods can be used to measure the amount of cylinder taper inside a cylinder: 1. One is to measure the amount of piston ring end gap in a cylinder. By measuring the end gap at the top of the cylinder, in the ring turnaround area, the middle, and the bottom, cylinder taper can be calculated. End gap is measured after “squaring” the ring inside the cylinder with a piston. A 0.001" (0.025 mm) increase in cylinder wall diameter will produce approximately a 0.003" (0.076 mm) increase in ring end gap. A typical maximum taper wear of a cylinder is 0.001" (0.025 mm) per inch (2.54 cm) of cylinder diameter up to 0.005" (0.127 mm) maximum wear in larger cylinders. 2. A quick and very effective method of evaluating excessive cylinder wear is to determine the percentage of missing cylinder wall crosshatch in comparison to the remaining

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Chapter 4 Cylinder Blocks and Crankshafts

crosshatch. The presence of crosshatch on the cylinder wall is important to provide a surface to seat piston rings and to retain oil for piston and ring lubrication. When crosshatch is absent, approximately 0.005" of wear has taken place. This means that highly polished walls without crosshatch indicate excessively worn cylinders, which are not capable of properly sealing the piston inside the cylinder bore. If 20% or more crosshatch is missing, the engine has 0% service life. Similarly, 10% missing crosshatch in the ring turnaround area of the cylinder wall indicates 50% or less remaining engine life. To visually evaluate cylinder wear, follow the steps in SKILL DRILL 4-1. 3. A dial bore gauge can be used to measure cylinder taper but not cylinder diameter. These gauges, which are dial indicators mounted on a bar that extends into the cylinder, are used when honing or performing other machining processes to ensure a symmetrical consistent dimension is produced (FIGURE 4-34). 4. Using a micrometer and inside telescoping gauges is another method. An inside telescoping gauge is used to gauge the cylinder diameter. An outside micrometer is then used to measure the width of the inside telescoping gauge.

SKILL DRILL 4-1 Visually Evaluating Cylinder Wear 1. One effective method to visually evaluate cylinder wear is to examine the ring turnaround area. 2. Using a piece of paper to reflect light directly into the crosshatch helps make the cylinder wear more visible. 3. If 20% or more of the crosshatch is missing, 0% service life remains. If 10% of the crosshatch is missing, 50% service life remains. An engine with a 5" (13 cm) stroke and 0.25" (6 mm) of polish has 75% service life remaining.

FIGURE 4-34 A dial bore gauge

is used to measure cylinder wear due to taper.

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Crankshafts: Construction and Function

▶▶ Crankshafts: Construction

and Function

The main purpose of the crankshaft in a reciprocating engine is to convert the linear motion of the connecting rod assemblies to rotational movement. While having a rugged function, the sophistication of the crankshaft is reflected in manufacturing costs, which makes the component one of the most expensive parts of an engine after the fuel system and turbocharger. The mechanical stress caused by rotational speeds, combustion pressure, thrust loads from the geartrain, and other mechanical vibrations require the crankshaft to be especially strong and precisely manufactured. Two connecting surfaces are the main and connecting rod bearing journals. Main bearing journals are located along the crankshaft centerline. Main bearing caps support the crankshaft as it rotates. Connecting rod journals are offset from the crankshaft centerline main journals. The distance of rod journal offset from the main bearing journals is known as the throw. The length of throw determines the stroke length of the engine. The arrangement of the rod journals, or throws, allows an even amount of spacing between power impulses. For example, a six-cylinder engine will have 120° of spacing between the rod journals (720° of rotation/six cylinders) (FIGURE 4-35). The throws are further arranged to evenly distribute the firing order over the length of the shaft to balance out vibration and stresses. For example, in in-line, six-cylinder engines, the throws of cylinder are paired 1-6, 2-5, 3-4. This distributes power impulses evenly throughout the crankshaft. When the engine is operating, tremendous inertial forces are generated by the reciprocating mass of the connecting rod and piston assembly. To balance these forces, large counterweights are formed on the crankshaft 180° opposite the rod journal (FIGURE 4-36). Fillet radii are another critical feature often unique to a diesel crankshaft. The fillet radius refers to the circular-shaped radius formed in the 90° intersection between the crankshafts rod and main bearing journals and the cheeks of the crankshaft. Torsional twisting of the crankshaft causes this area to be one of the most stressed areas of the crankshaft. Leaving only a sharp 90° angle between the two surfaces could induce a crack through a stress riser. Imparting a smooth circular radius to this area strengthens the crankshaft and minimizes the possibility of a fracture.

4-06 Identify and explain the purpose of unique features used to construct diesel engine crankshafts.

4 1

1-5

120° 180°

120°

90°

180°

2 3

4-8

2-6

3 2 Four Cylinder 1

3-7

90°

3 120°

120° Inline Six Cylinder

1-4

3-6

2-5

90°

5 2

FIGURE 4-35 Arrangement of crankpins spaces firing impulses evenly. In-line, six-cylinder engines space crank throws at 120°.

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Chapter 4 Cylinder Blocks and Crankshafts Balancing Hole Counterweight Oil Passage

Crank Snout

Flywheel Flange

Crank Cheeks Connecting Rod Journal

Main Bearing Journals Fillet Radius

Counterweights FIGURE 4-36 Crankshaft terminology. Crankshaft counterweights offset the mass of the connecting rod and piston assemblies.

Crankshaft Journal Hardening Crankshafts in diesel engines are commonly forged from high carbon steel alloys because forging produces a denser and stronger component. Following the machining of crankpins, counterweights, and other surfaces, journal surfaces are hardened to improve the durability of a diesel crankshaft. The two most common processes used to harden bearing surfaces of the crankshaft are induction hardening and nitriding.

Induction Hardening

FIGURE 4-37 Induction hardening causes discoloration of metal

surfaces hardened through this process. This crankshaft has discoloration around hardened journals, which is not evidence of damage from overheating but is caused by heating during induction hardening.

To induction harden a crankshaft, the journal is wrapped in a coil of wire that has high amperage, alternating current passed through it. The journal surface will have current induced into it by mutual induction, much like the secondary winding of an ignition coil. This current flows over the journal surface, heating the metal surface and changing its molecular structure (FIGURE 4-37). Water quenching the heated surfaces produces a very hard surface finish. Typically, induction hardening is effective to no more than 0.030" (0.762 mm) deep. It may go deeper, but it becomes softer with increasing depth. This is one reason that explains why machining diesel crankshafts is not a good work practice: The hardening material is lost and most rebuilders are not able to restore it during rebuilding and repair. Spray welding the crankshaft with high alloy steel is a more effective technique for crankshaft restoration. This process is performed by the OEM by using advanced techniques and facilities.

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Crankshafts: Construction and Function

Nitriding Another crankshaft hardening technique is nitriding. Chevy diesels, which include the Duramax and the older 6.2 L and 6.5 L, use this technique. The process involves heating the shaft and introducing cyanide salt or liquid (CN3) into the heating chamber with the crankshaft. Slowly cooling or quenching the crank with these substances in its heated condition causes the nitrogen and carbon to molecularly bond with metal surfaces to create an extremely hard journal. This hardening finish does not extend more than 0.012" (0.305 mm) deep but is one of the hardest journal surface finishing techniques.

Crankshaft Lubrication To lubricate the main and rod bearings, engine oil needs to be directed in sufficient quantities to lubricate, cool, and clean the crankshaft journals and bearings. Engine oil is delivered to the oil galleries in the block and then distributed to the main bearings by using holes present in the upper main bearing shells. A cross-drilled hole in the crank main bearing journal uniformly distributes oil around the crankshaft journal (FIGURE4-38). From there, the crankshaft has holes drilled from main bearing cross-drilling to the rod journal. Cross-drilling rod and main bearing journals means each main bearing is connected to an adjacent rod bearing through an oil passageway. The exception to this would be the rear main, which in some cases has a smaller oil hole into the block only for lubrication of one main journal (FIGURE 4-39). The requirement to supply only one main bearing journal relieves lubrication oil pressure exerted against the rear main seal, which minimizes potential leaks and oil weepage from the rear main seal.

101

▶▶TECHNICIAN TIP Grinding diesel crankshafts to undersize dimensions to repair damaged journal surfaces is common in gasoline-fueled engines. Diesel cranks, however, are almost always hardened, and grinding to repair journal damage will remove the surface hardening. Furthermore, a fillet radius ground into each crankshaft journal is difficult for most machine shops to restore. For these reasons and because most manufacturers do not recommend the practice, undersize crankshaft bearings are not commonly available for diesel crankshafts.

Connecting Rod Oil Passages

Cylinder Block

From

Oil Passage

Crankpin Journal

Oil Pump

Crankshaft Oil Passages

Main Bearing Journal

FIGURE 4-38 Drilled passageways through the crankshaft distributes pressurized oil to the rod bearings and piston pins in

some engines. Passageways drilled between the oil rail and the main bearing supply oil to the crankshaft.

FIGURE 4-39 The location

of a rear main seal, which prevents oil from leaking out of the crankcase.

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Chapter 4 Cylinder Blocks and Crankshafts

Crankshafts and Engine Balance As noted in previous paragraphs outlining techniques to construct strong durable block designs, it’s important to understand that combustion forces operate to push apart the pistons and crankshaft from the block. Understanding this concept helps explain why in-line engines are preferred in vocational and heavy-duty diesel applications since the crankshaft is supported by more main bearings than in V-type configurations (seven main bearings in in-line, six-cylinder vs. four in a V-banked cylinder engine). This means the in-line block is better able to handle higher power output with greater durability than V-block configurations. The advantage of a V-block is that it permits lower hood profiles due to a more compact engine design. The dilemma iOEM’s have selecting light duty diesels such as Ford’s 3.0 L V-6 and GM’s 3.0 L inline 6-cylinder 3.0 L is choosing between an engine that can be packaged better for use in a vareity of chassis’ or choosing a block configuration with potentially greater durability and power output. A V-6 can be installed longitudinally or transversely in rear or front wheel drive vehicles. An inline engine can generally fit into only a rear wheel drive pick-up or large delivery van. Cylinder banks leaned 60°, 72°, or 90° result in an engine with less overall height (FIGURE 4-40). The spacing of cylinder banks also helps reduce imbalance forces in engines caused by the spacing of firing impulses. For example, firing impulses of a V8 engine occurring every 90° of crank rotation are evenly spaced 90° apart in cylinder banks with 90° spacing. In V6 configurations, a 60° bank spacing separates firing impulses taking place every 120° by an even 120° of crankshaft rotation. Mercedes-Benz uses a 72° bank spacing, plus the use of a counter-rotating balance shaft in between the cylinder banks for its Bluetech 3.0 L engine. The crankpins are offset by 48° to eliminate vibration problems from uneven spacing between combustion events, making the engine evenly spaced when firing with 72° bank spacing.

Primary Balance Primary balance is achieved when the crankshaft counterweights offset the weight of the piston and connecting rod assembly. Most engines use a counterweight on the crankshaft for each cylinder to counteract the inertia of the piston and connecting rod assemblies to minimize engine vibration. In in-line, six-cylinder engines, crankshaft counterweights can be removed from cylinders 2 and 5 since the inertia forces generated by other piston pairs will cancel imbalance forces. Externally balanced engines will use weights on flywheels or ring gears to help achieve good primary balance.

Secondary Balance Secondary balance is achieved when the inertia forces of one piston counterbalances the inertia of another (FIGURE 4-41). Since all the pistons are in one plane in in-line engines, secondary balance is easily achieved. Inherent in the in-line, six-cylinder design is the advantage pairing pistons, which move simultaneously in the same direction and plane against those moving in the opposite direction. The dynamic forces of piston weight can be canceled out by the forces of pistons moving in the opposite direction. Horizontally opposed engines also have the best secondary balance. In contrast, V-engines have a natural secondary imbalance due to the pistons operating in different planes. Piston movement may cause engine vibration because pistons do not move in the same plane simultaneously. The direction of movement may be 60°, 72°, or 90° apart. An in-line, six-cylinder engine is 60°

72°

90°

FIGURE 4-40 Common cylinder bank spacing arrangements.

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Vibration Dampeners

Top Dead Center 1

103

Inertial Forces

V6 Engine Bank

Front Cylinders

Rear Cylinders

1 Inertial Forces

6 2

Top Dead Center 120° and 240° After Top Dead Center Bottom Dead Center

Inline Six-Cylinder Engine 5

FIGURE 4-41 Secondary balance requires the inertia forces of piston pairs to cancel out the inertia

L 8

of pistons traveling in the opposite direction.

also among the most balanced configurations for the internal combustion engine, possessing excellent primary and secondary balance characteristics.

Firing Orders Firing orders and cylinder numbering are important to technicians when locating cylinders with fault codes associated with their operation. Cylinder numbering begins from the front of the engine with the most forward cylinder being the number 1 cylinder and ends at the rear cylinder with the last number in the firing order. The leading bank of a V-type engine identifies cylinder 1 (FIGURE 4-42). Since the flywheel is the reference point for engine rotation, the front of the engine is opposite the flywheel. Firing orders are also arranged to produce even spacing between power impulses and to distribute loads evenly along the crankshaft. This arrangement minimizes damaging torsional or twisting forces along the crankshaft. An even number of degrees of crankshaft rotation between cylinder power impulses minimizes engine vibration.

▶▶ Vibration

4 2

DMAX 1-2-7-8-4-5-6-3 International 1-2-7-3-4-5-6-8 FIGURE 4-42 Firing order of a

Duramax V8.

Dampeners

Two potentially destructive forces applied to a crankshaft are harmonic and torsional vibration. As previously mentioned, torsional vibration is observed as the deceleration and acceleration of the crankshaft during rotation and is caused by the alternating combustion and compression pressure in each cylinder. At specific engine speeds, accessory drive belts can be spontaneously thrown and driveline vibration can occur due to excessive, uncontrolled torsional vibration. Alternators, air conditioning compressors, and other accessories will loosen and fall off due to excessive torsional twisting of the crankshaft transmitted to components through the gear train and drive belts. Cylinder pressure applied against the piston and connecting rod assembly is also intense enough to deflect and twist the crankshaft while it turns. The twisting event eventually results in a rebound of the crankshaft as the

4-07 Identify and explain the function of vibration dampeners.

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Chapter 4 Cylinder Blocks and Crankshafts

journal snaps back in the opposite direction of rotation. The effect of this harmonic stress is telegraphed through the geartrain and driveline, and Crankshaft if not properly controlled, torsional vibration causes rapid main bearing and geartrain wear and possibly causes the crankshaft and camshaft to break as both torsional and harmonic to forces are transmitted through the geartrain. Harmonic vibration or waves are like the plucking and Inertial Twist subsequent vibration of a guitar string. The frequency of these harmonic vibrations allows pressure waves to be transmitted back and forth along the length of the crank, much like an instrument string or a long wire of a clothesline. Nodal points—the place where the amplitude of the wave Crankpin Journal form is reversed—are formed by harmonic waves along the crank and Main Bearing Journal are points of potential failures due to harmonic vibration (FIGURE 4-43). To control the destructive effects of both torsional and harmonic vibrations, diesel engines are equipped with a vibration dampening devices, which reduce both torsional and harmonic vibrations. The mass Rebound of the flywheel can substantially suppress torsional vibration at its end of the crankshaft. At the flywheel, changes in crankshaft velocity are miniTwist mized by the inertia of the flywheel that resists speeding up and slowing down with each change in cylinder pressure. The large diameter of the flywheel dampens harmonic waves, which tend to bend the crankshaft away FIGURE 4-43 Harmonic twisting is different from torsional from the crankshaft centerline. To reduce torsional and harmonic vibratvibration. A crankshaft will actually deflect under power impulses, ing forces at the opposite end of crank, a vibration damper is located at the producing harmonic wave. Torsional vibration involves the front of the crank. Like the dampening of a soft glove on a ringing metal rhythmic speeding up and slowing down of the entire crankshaft. bell, the front vibration dampener quickly absorbs harmonic vibrations. Balance shafts are another vibrating dampening mechanism used to cancel out rhythmic forces produced by long or uneven delays between firing impulses from an engine. In smaller three- or four-cylinder engines, the torsional vibrations are more pronounced since the crankshaft is allowed more time to decelerate between firing impulses happening every 180° or longer in comparison to 120° in six-cylinder engines or 90° in V8 engines. Balance shafts can also help counterbalance the fore and aft engine movement in V-6 engines casued by the inertia forces of pistons changing direction in an engine (FIGURE 4-44). The following are four types of dampeners used on diesel engines: Power Stroke

1. elastomer dampeners 2. viscous or fluid dampeners 3. combination dampeners 4. rotating pendulum vibration absorbers.

Crankshaft Balance Shafts FIGURE 4-44 Balance shafts help

correct for secondary imbalances caused by changes in piston direction.

Vibration Direction

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Vibration Dampeners

Elastomeric Dampeners Elastomer dampeners refer to vibration dampeners using rubber as the primary material to absorb harmonic vibrations and reduce torsional stress on the crankshaft (FIGURE 4-45). These are made of three parts. The inner hub is attached to the crankshaft, which connects to a heavier, outer iron inertia ring through a rubber or elastomeric ring. The dampening action occurs as the outer ring oscillates with speed changes relative to the inner ring movement. For example, when the crankshaft slows, the inner ring is sped up by the inertia of the outer ring. This means that when the inner ring slows down, the outer ring transmits inertial force through the rubber ring to speed up the inner ring (FIGURE 4-46). The opposite action occurs as the inner ring accelerates and resistance to the sudden speed change is transmitted through the rubber ring. The back and forth movement of the two rings out of phase with one another results in an elastomeric dampener naturally tuned to vibrate out of phase with the lowest torsional frequency of the engine. Alignment and verification of the proper operation of the two inertia rings is confirmed by the use of a mark scribed in the face of the outer and inner ring. If the marks are not aligned, the two rings have debonded from the elastomer ring, so the dampener is defective (FIGURE 4-47).

Viscous Dampeners

105

▶▶TECHNICIAN TIP Dampeners should never be removed with a puller applied to the outer inertia ring. Most dampeners are designed to be removed with hand force only. Pulling on the outer ring or the case of a viscous dampener will prevent the dampener from functioning properly.

▶▶TECHNICIAN TIP Timing marks located on the outside of elastomer vibration dampeners may be incorrect if the outer hub has slipped relative to the inner hub. Oil on the dampener and age are common factors causing slippage, so a technician must carefully inspect the dampener for evidence of slippage.

Silicone fluid is used in viscous dampeners to tune out harmonic and torsional vibrations. Higher horsepower (kilowatt) engines, those over 300 hp (223.7 kW), will possess higher vibrational energy, which can lead to overheating and shortening of an elastomer dampener’s life. In a viscous damper, the inertia ring rotates inside of a sealed case connected to the crankshaft. A thin film of highly viscous silicone fluid fills the small gap between the inner inertia ring and the outer case (Figure 4-47). During changes in crankshaft speed, the silicone fluid drags the inertia ring, permitting it to rotate at approximately the same rpm as the crankshaft. If no torsional vibration is present, the inertia ring and outer casing will rotate at identical speeds. However, changes to crankshaft speed due to torsional vibrations cause the inertia ring and outer case to shear the silicone fluid. This converts the relative motion between the inertia ring and the outer case into heat energy and is effective at dampening torsional and harmonic vibrations caused by defection over the face of the inertia ring. The oscillating (back-and-forth) movement between the ring and the case allows the viscous dampener to dissipate more energy than the same size elastomer-type damper, permitting more effective dampening of torsional vibrations.

Outer Member

Alignment Marks

Rubber Insert

Inner Member

FIGURE 4-46 How the elastomeric-type vibration dampener FIGURE 4-45 This elastomeric vibration damper from a 3.2 L Ford

diesel has two rubber elastomeric bands to balance torsional vibration from 5 cylinders.

operates. The outer inertia ring and inner hub move in opposite directions during a torsional impulse. The movements help minimize changes to crankshaft speed by accelerating the crank when it slows and by slowing it down when it speeds up.

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Chapter 4 Cylinder Blocks and Crankshafts

Outer Inertia Ring

Outer Ring

Index Marks Elastomer Ring

Inner Inertia Ring

Inner Hub

FIGURE 4-47 Elastomeric- and viscous-type dampeners. Lines on the outer and inner hub of an elastomer dampener should

always line up. If they don’t, the dampener is defective.

The shearing effect of silicone fluid actually produces heat that can be used to quickly check the viscous dampener operation. After running an engine for 10–20 minutes, the case should feel quite warm to touch if the viscous dampener is operating correctly.

Combination Dampeners Combination dampeners combine the dampening advantages of both viscous and elastomeric dampeners. Elastomeric-type dampeners provide good low-speed harmonic and torsional dampening in comparison to viscous-style dampeners. However, at higher engine speeds, the reverse is true, where viscous dampeners provide better vibration dampening.

Rotating Pendulum Vibration Absorber Unlike regular balancers, pendulum dampeners provide torsional control by producing forces that directly cancel the forces producing torsional vibration. Steel rollers, called centrifugal pendulums, fit loosely into a specific number of holes in either a harmonic balancer or an overhead cam drive gear. The rollers store and release energy back into the crankshaft rather than convert the mechanical energy into heat energy as dampers do. An engine specific mathematical algorithm is used to calculate roller size, hole size, and gear size regarding how the rollers will move forward during compression strokes when the crank slows and rolls backward during the power stroke. The back-and-forth movement of the rollers minimizes engine speed changes, keeping torsional vibration low. Manufacturers claim that the pendulum dampener provides an engine that runs more smoothly, increases valve train stability while offering more accurate injection timing. Pendulum dampeners also have the advantage of being lighter than conventional dampeners and can be used on either the crankshaft or camshaft of overhead cam engines.

Balance Shafts Balance shafts are shafts with eccentric weights attached to rotating shafts that are used to dampen engine vibration caused by the inertia forces of pistons changing direction at top dead center (TDC) and bottom dead center (BDC). Because the inertia force is generated twice in every stroke, the vibration occurs at twice the engine speed. This explains why balance shafts are driven at two times engine speed and when two balance shafts are used, they rotate in opposite directions of one another at twice the engine speed, countering secondary imbalance forces. Balance shafts help eliminate vibrations that drivers and passengers might otherwise feel transmitted through the engine mounts to the steering wheel, seats, floor pan, or instrument panel. Three- and four-cylinder engines commonly use balance shafts. But V-6 engines can benefit from a fore and aft or front to back vibration caused by pistons changing over at TDC at slightly different times and in different planes.

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Flywheels

107

▶▶ Flywheels Flywheels are like large plates bolted to the rear of the engine, and they serve several purposes. Their primary purpose is to provide some inertia between power impulses to keep the engine rotating. Storing energy in the heavy flywheel produced during each power impulse helps smooth out engine speed. A toothed ring gear fastened to the circumference of the flywheel provides a drive point for the starting motor pinion gear. To crank the engine, the starter will engage the engine through the flywheel ring gear. Turning tools are often temporarily engaged with the flywheel to rotate the engine when performing service procedures. A flywheel also provides a mounting surface for a clutch in the case of a manual transmission and a torque converter for automatic transmissions. Flywheel shape and construction can vary, depending on the type of clutch used and whether torsional dampening capabilities are built into the flywheel.

4-08 Identify flywheel types and construction features.

Dual Mass Flywheels Torsional vibration dampening is important to drivetrain durability and reduction of noise, vibration, and harshness. The GM 6.2 L; the Ford 6.0 L, 6.9 L, 7.3 L, and 7.3 L Powerstroke; and some engines manufactured by Volkswagen, BMW, and others incorporate specialized torsional dampening springs in their construction. Known as dual mass flywheels (DMFs), when connected to manual transmissions, they enable the use of lighter transmissions and drivelines by minimizing torque spikes produced by torsional vibration (FIGURE 4-48). Transmissions shift more easily and transmission gear rattle is virtually eliminated with DMFs. DMFs are made from two separate sections, which can be disassembled and inspected during clutch servicing. The clutch surface and ring gear are each separate halves of the flywheel. DMFs can also be recognized by a series of springs located around the flywheel circumference on the engine side of the flywheel (FIGURE 4-49). One set of bolts near the center hub connects the flywheel to the crankshaft, and another set holds both halves of the flywheel together. Secondary Flywheel Mass Primary Flywheel Mass

Clutch Disc with Dampening Springs

Vibrations from the Engine Flywheel

Clutch Disc with Torsion Damper

Vibrations Absorbed by the Gearbox

FIGURE 4-48 DMFs minimize the transfer of torsional vibration to transmissions and drivelines.

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Chapter 4 Cylinder Blocks and Crankshafts Crankshaft Driven Disc

Torsional Dampening

Front Clutch

Springs

Disc Driven by Crank Disc

Friction Ring Material FIGURE 4-49 (Rear Secondary Plate) A DMF is constructed

of two major parts. This rear section contains dampening springs used to absorb torsional vibration. Energy from the crankshaft is transmitted through the springs and into the friction material on the face of this section.

FIGURE 4-50 (Front Primary) A flat steel clutch plate (removed)

bolts to this section front section of the flywheel. Torque is not transmitted directly from the crankshaft to the clutch plate. Instead, torsional dampening springs transmit torque from the outer disc to the inner disc and then to the clutch plate.

Power is transmitted to the flywheel clutch mounting surface through a series of springs located around the circumference of the rear plate, which attaches to the crankshaft. These springs act like the dampening springs of a clutch disc to reduce torsional vibration. On the front clutch disc is friction material that transmits drive torque to the front clutch disc from the rear disc through torsional dampening springs. This friction ring will slip under high torque input, which further prevents transmission damage from excessively high torque inputs caused by loading the vehicle beyond its gross vehicle weight (GVW) capacity (FIGURE 4-50). The friction ring is made of a material similar to brake lining and can wear out if excessive torque loads are continuously applied. If the customer complains of any unexplained engine noises, vibration, or clutch slippage, then the possibility of a defective DMF flywheel should be investigated. Friction material between the flywheel plates could be burned, and springs could be broken, missing, or worn out. To inspect a DMF, follow the steps in SKILL DRILL 4-2.

SKILL DRILL 4-2 Inspecting a Dual Mass Flywheel

1. Operate the engine and listen for unusual noises originating from the flywheel. Pay particular attention to flywheel spring rattling and clunking sounds during engine start-up or shutoff. Also listen for excessive gear rattle 2. After removing the clutch from the flywheel, unbolt the DMF clutch plate cover and check the drive plate for any damage.

Pay attention to straps and the friction material. Recommend replacement if the friction material is burned or glazed or if any springs are missing. 3. Inspect the spring retainer for damage and recommend replacement if large gaps exist between the springs and nylon retainers. 4. Reinstall the clutch cover and measure the flywheel free play. This can be done by turning the flywheel clutch cover all the way to a stop point in either a clockwise or counterclockwise rotation. Mark the flywheel cover and line up the mark with one of the starter’s ring gear teeth. Rotate the clutch cover again, and mark the limit of flywheel rotation by lining up the mark on the cover with another ring gear tooth. Continued

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Engine Bearings

5. Count the number of teeth the clutch cover moved and compare with specifications. There are specifications for each flywheel, but generally, more than 1" of free play travel indicates that replacement is required. 6. Check for secondary plate rock. This test checks the axial movement. The secondary plate is the plate behind the clutch

▶▶ Engine

109

cover plate and no axial movement should be observed on the secondary plate. Push on one edge of the plate. Check to see whether the opposite side also moves away when one side is pushed. If it does, the flywheel is defective.

Bearings

Engine bearings are replaceable wear surfaces used on the rotating journals of the crankshaft and camshaft. Bearings used on the crankshaft journals are referred to as main bearings. The upper shell of a main bearing always has a hole to receive lubrication and a circumferential groove to channel oil from the oil supply hole to the rest of the bearing. Rod bearings are used on the crankpin journals or connecting rod journal surfaces (FIGURE 4-51). The engine’s connecting rods and main bearing caps could be designed to operate without bearings, but those parts would require much more expensive and intensive labor to replace. The materials that bearings are made from are also chosen to extend the life of the engine by reducing friction between the moving surfaces. Experimentation with various types of bearing materials and construction techniques have helped diesel engines evolve to the point where more than 1.5 million miles (2,414,000 km) can accumulate on heavy-duty diesels before the bearings need replacement. Hardened bearing shells, called sputter bearings, are the latest bearing technology used to extend bearing life even further (FIGURE 4-52). Sputter bearings use a construction technique that hardens the bearings top overlay surface to increase their durability. A much harder layer of aluminum and tim is applied using Physical Vapor Deposition (PVD) technique. The hardened sputter overlay, which may have a blackened or red oxide appearance, is much harder than traditional lead-tin alloys. The hardness of these bearings is 3 times greater than conventional lead-tin alloys. Loading pressures are the highest of any bearing having up to 100-120 MPa (14500-17400 psi) load bearing capacity. Operating clearances on these bearings are also much smaller—typically just over 0.001" (0.025 mm) of oil film compared to as much as 0.0035" (0.089 mm) in conventional bearings. Hardness of aluminum-tin sputter material is about 90 HV, which is three times higher than hardness of aluminum-tin alloy prepared by conventional methods (casting). Cast copper based bearings or high strength aluminum based bearings are commonly plated by sputter overlays. Load carrying capacity of sputter bearings is highest of all bearing materials, being in the region of 100-120 MPa (14500-17400 psi). While bearings have increased engine longevity, technicians still need to be aware of different types of bearings and their construction, to be ready to diagnose problems with bearings and replace them when necessary.

4-09 Identify and explain the relationship of construction features to the functions of engine bearings.

▶▶TECHNICIAN TIP The latest engine bearing called sputter bearings use hardened bearing surfaces and operate with much smaller oil clearances. It is imperative that low viscosity oils such as 5w-30 or 10w-30 FE grade oils only are used with these bearings. Using thicker, higher viscosity oils will potentially cause oil starvation leading to catastrophic engine damage.

Main Bearings

Locating Lug

Upper Bearing Locating Lug Half Oil Hole Spreader Groove, Oil Groove

Connecting Rod Bearing Locating Lug Upper Bearing Half Oil Hole

Mating Faces

FIGURE 4-51 Rods and main Length Crown Area

Lower Bearing Half

Mating Faces Locating Lug Oil Hole Crown Area

Length Lower Bearing Half

bearings are constructed differently. An oil hole with an annular groove to distribute oil is typical for the upper main bearing shell. An oil hole may be present in the rod bearing if pressurized oil is used to lubricate the piston pin.

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Chapter 4 Cylinder Blocks and Crankshafts

Plain Insert Bearing Engine bearings are known by a number of names: Precision inserts, plain bearings, and tri-metal bearings are a few common ones. Bearings used around the crankshaft are split-type insert bearings because the bearing is made in two pieces. Manufacturing the bearing into halves allows the bearing to be easily serviced and allows the use of a one-piece crankshaft. Camshaft bushings are examples of plain bearings that are one piece. Plain bearings also are lighter and more compact for their load bearing capacity than roller types are. Bearings perform a number of important functions inside the engine: ■■ ■■ ■■

FIGURE 4-52 Sputter bearings use a hardened bearing shell.

The darker bearing shell is hardened.

Heavy Pressure

Oil Film

Bearings Crankpin Journal Rotation

Pressure Area

Oil Wedge

FIGURE 4-53 Formation of the hydrodynamic wedge.

reducing friction supporting moving parts under load serving as replaceable wear surfaces.

Reducing friction is one of the most obvious functions that bearings perform, along with the lubrication system. Bearing surfaces should minimize friction and heat generation. It should be noted that plain bearings with a pressurized oil film have less high-speed friction than an antifriction type (roller, ball bearing, etc.). To achieve the engine life that bearings have today, plain bearings are the best to use. A Cummins 6.7 L has a B-50 life of 350,000 miles (~550,000 kms). That means that at that distance there is still 50% bearing life remaining. Fords 6.7 L V8 B10 bearing life gives it’s engines a 90% probability of operating an to 500,000 miles (~800,00 kms) without requiring the removal of the cylinder heads or oil pan for service. To achieve the long durability the characteristics of the lubrication film between the rotating shaft and bearing are critical. The term “hydrodynamic wedge” is the explanation given to understand why bearings can last so long but wear so little. Essentially, the oil, bearing, and rotating shaft are separated from one another by hydraulic pressure. Engine bearings depend on a film of oil to keep the shaft and bearing surfaces separated (FIGURE 4-53). Bearings fail when the oil film breaks down or when the bearing is overloaded. The oil film pressure is generated by shaft rotation. At rest, the shaft and bearing are in contact. On start-up, the shaft contacts the bearing briefly. As it runs, the shaft pulls oil from the clearance space into the wedge-shaped area between the shaft and bearing. Oil is pulled in because of the attraction oil has to the metal shaft. The oil film pressure is generated by shaft rotation to wedge or lift the shaft off its bearing shell and support it during engine operation. During normal operating conditions, a continuous supply of clean oil will keep the shaft and bearing surfaces separated. Bearings fail when the oil film breaks down or when the bearing is overloaded. At rest, the shaft and bearing are in contact. On start-up the shaft may briefly contact the bearing. While running, the shaft pulls oil from the clearance space into the wedge shape area between the shaft and bearing. Oil is pulled in because of the electrostatic attraction oil has to the metal shaft. The oil wedge lifts the shaft off its bearing and supports it during engine operation. During normal operating conditions, a continuous supply of clean oil between the shaft and bearing surfaces will ensure that the hydrodynamic wedge separates the two surfaces.

Bearing Construction In early automotive history, bearings were made of a lead-tin alloy called “Babbitt.” This alloy was effective because of its low frictional characteristics: the ability to distort under severe load and embed dirt into its shell so that the crankshaft would not be damaged. However, one characteristic these bearings did not have was fatigue strength: the ability to withstand high loading for long periods of time.

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Engine Bearings

The following is a summary of those important bearing characteristics: ■■

■■

Compatibility: a bearing’s ability to allow friction without excessive wear or friction. Dissimilar metals have better compatibility. Copper to steel and bronze or lead to steel have good compatibility or lower wear when used against one another. A bronze crankshaft with steel bearings could be combined to minimize wear, except the crank isn’t as strong as steel. Fatigue strength: a bearing’s ability to carry a load. A bearing will crack or be wiped away if overloaded by shaft pressure. Conformability: a bearing’s ability to conform to irregularities on of a journal surface (which works against fatigue strength). Embeddability: a bearing’s ability to absorb particle contamination. Dirt particles will scratch a shaft and ruin it (FIGURE 4-54).

A variety of construction techniques are used to achieve different balances among the characteristics that will adapt to each engine’s unique operating conditions. Tri-metal bearings with copper alloys for the intermediate layer are used in diesel engines. A steel back with a copper alloy intermediate layer gives the strength, fatigue resistance, and conformability characteristics required for a diesel engine bearing. The lead-tin Babbitt layer provides good wear and compatibility features. A nickel barrier plating prevents galvanic reactions between the Babbitt and the copper alloy surface beneath it that would lead to corrosion. Flash lead-tin plating protects and provides a finish for the engine break-in period. This finish can be easily removed when it’s new, by touching the bearing or wiping it with a shop rag (FIGURE 4-55).

111

Dirt Particle Bearing Lining

Displaced Bearing Material Oil Clearance

Bearing Backing

Journal

FIGURE 4-54 Embeddability refers to the ability of dirt particles

to embed themselves into the soft lead layer of Babbitt to prevent scoring the crank journal. Nickel Dam Copper/Lead Alloy

Steel Backing Lead/Tin/Copper Overlay FIGURE 4-55 A tri-metal plain insert bearing is made up of three

layers of metal with a nickel layer between each.

Bearing Lubrication Every bearing receives lubrication through holes drilled into the crankshaft. Main bearings receive oil first from the oil pump and main oil gallery. The oil enters the main bearing journal and drilled passages inside the crankshaft to flow to the rod bearing journals and bearings. Some main bearing journals are cross-drilled so that the oil holes align twice for every crankshaft revolution. Oil would otherwise be cut off, so the main bearings have a groove down the center of the top bearing shell that allows oil to flow around the main bearing journal. The amount of oil clearance is important to maintaining the hydraulic pressure of the oil wedge separating the rotating shaft and bearing. Too little clearance, and inadequate oil will flow into the bearing for lubrication and cooling. Too much clearance, and the oil wedge will collapse, resulting in shaft to bearing contact. Excessive bearing clearance is a primary reason to have low oil pressure. If the oil pump is required to pump extra volume to fill the clearances while even greater quantities of oil are thrown of a shaft, loss of engine oil pressure will result. (recall that pumps produce volume, not pressure; lubrication systems require restrictions to produce pressure.) Higher shaft speeds also produce greater oil wedge pressure.

Plastigauging To evaluate the oil clearances of engine bearings, the use of Plastigauge is recommended. Plastigauge consists of two essential parts: ■■ ■■

an oil-soluble plastic material packaging with a printed thickness gauge.

Measuring bearing clearances with Plastigauge is very simple when the engine removed an is turned upside down while on an engine stand. After the main bearing cap has been removed and the oil wiped from the crankshaft and bearing shell, a piece of Plastigauge is laid across the crankshaft journal. The bearing cap is reinstalled with a torque recommended by the engine manufacturer. When the bearing cap is tightened,

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Chapter 4 Cylinder Blocks and Crankshafts

FIGURE 4-56 Plastigauging a main bearing journal determines oil clearances and is typically done only when a replacement crank or service crank

is used. The Plastigauge is squeezed by the bearing and cap. Afterward, the width of the Plastigauge is compared against a gauge on the package.

the pressure causes the Plastigauge to be flattened. The less clearance there is, the greater the flattening and the wider it will be. When the bearing cap is removed, the width is measured by direct comparison with the graduated scale on the Plastigauge envelope. The numbers on the graduated scale indicate bearing clearance in thousandths of an inch or millimeter on the reverse side of the paper gauge. Plastigauge is available in four different sizes and differentiated by four colors—red, green, yellow, and blue—each covering a particular bearing clearance range (FIGURE 4-56).

Bearing Location Bearings must not move within the bearing bore and must have good contact within the bore for good heat transfer. Several bearing construction features are common to ensure these functions. Bearing spread, refers to the parting face of a bearing being made wider than the diameter of the cap or web, assists in bearing retention and ensuring good contact with the cap or web of the bearing bore for heat transfer. Bearings require being “snapped” or pushed into place (FIGURE 4-57). Bearing crush is the distance that a bearing is higher than the cap or web. In fact, the diameter of the bearing shells is larger than the bearing bore. This feature assists in locking the bearing in place and heat transfer from the bearing shell into the bearing bore. The dark spots observed on the back of a bearing indicate where greatest heat transfer takes place. For this reason, bearings should always be installed dry on the backside with no grease or threadlockers applied to the back of the shell. Using this material would impede the transfer A Is Greater than B A

Bearing Shell Higher than Cap

Bearing Shell

Bearing Cap

Bearing Cap Bearing Shell Full Seated in Cap

FIGURE 4-57 The concepts of bearing crush and spread. Bearings must seat tightly into the bearing bore to

prevent movement and to transfer heat.

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Engine Bearings

113

of heat from the bearing. Locating lugs are used on bearings to prevent movement and to properly center the bearing in the bore. To ensure that bearing caps and lugs are not incorrectly matched, assemble the caps together, locating tab to tab. Make sure identification numbers match for main bearing caps location in block and on connecting rods to caps.

Undersize and Oversize Bearings An undersize bearing is thicker than a standard bearing. It does not describe the bearing but the shaft it fits. If a crankshaft is worn or is ground to an undersize, an undersize bearing is used to fit the journal. If a spun main bearing damages a block, the bearing bore can be re-bored to an oversize and an oversized bearing can be installed. Cam bearings can typically outlast two sets of mains since there is less loading; however, excessively worn and bleeding cam bearings with excessive oil throw-off can starve mains.

FIGURE 4-58 A thrust bearing is used on a main

bearing journal to control crankshaft endplay.

Thrust Bearings Thrust bearings control end play in a crankshaft (FIGURE 4-58). The forces of disengaging the clutch, helical gearing on the geartrain, and thrust from automatic torque converters can easily push/pull a crankshaft to the point where severe damage to the connecting rods and journals occur. Thrust bearings are used to control end play (FIGURE 4-59). Washers or bearings with flanges ride against a specially machined journal on the cheeks of only one main bearing journal. To measure crankshaft end play, follow the steps in SKILL DRILL 4-3.

Bearing Failures Bearing failures are indicated by the following conditions: ■■ ■■ ■■

a drop in lubricating oil pressure excessive oil consumption (excessive oil thrown onto the cylinder walls) noises that can be minimized during a cylinder cutout procedure (FIGURE 4-60).

FIGURE 4-59 Measuring crankshaft end play with a dial

indicator. The crank is pushed all the way back, the dial indicator zeroed, and then the dial is pushed forward to measure travel distance.

2.8% Storage and Handling 6.9% Overloading 17.7% Installation Errors 18.6% Other Causes 19.9% Contaminated Lubricant 34% Inadequate Lubrication

FIGURE 4-60 Causes of

bearing failures.

SKILL DRILL 4-3 Measuring Crankshaft End Play 1. Place a dial indicator magnetic base on the engine block. Place the needle indicator on the vibration dampener. 2. If the vehicle is equipped with a manual transmission, push on the clutch. With the clutch depressed, zero the dial indicator. Alternatively, a pry bar between the flywheel and flywheel bell housing inspection hole can be used to pull the flywheel rearward. 3. Push the flywheel forward using a pry bar between the flywheel and the inspection hole in the flywheel housing. 4. Observe and record the measurement on the dial indicator. Specifications are generally in the range of 0.004–0.019" (0.102–0.483 mm). The specifications may seem large, but bearing clearances must be wide enough to allow oil to flow on both sides of two thrust washers or bearing flanges.

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Chapter 4 Cylinder Blocks and Crankshafts

▶▶Wrap-Up Ready for Review ▶▶

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▶▶

The cylinder block is the largest single part and main structure of the diesel engine, and its primary function is to support cylinders and liners (if equipped) and major engine components, including the crankshaft and camshaft. Diesel engine blocks have cylinders arranged in-line, V-banked, or horizontally opposed and can be configured with different numbers of cylinder bores. Cylinder blocks can be constructed from cast iron, aluminum, or compacted graphite iron (CGI); each of these materials has advantages and disadvantages. Cylinder block walls progressively wear with increasing engine hours and distance traveled; a dial bore gauge is used to measure cylinder wear due to taper. The main purpose of the crankshaft in a reciprocating engine is to convert the linear motion of the connecting rod assemblies to rotational movement. Primary balance, which is achieved when the crankshaft counterweights offset the weight of the piston and connecting rod assembly, and secondary balance, which is achieved when the movement of one piston counterbalances the movement of another, result in engines that run more smoothly. Counter balance shafts are used more commonly in threeand four-cylinder engines to compensate for engine vibration produced by changes in the direction of pistons. The mechanical stresses placed on a crankshaft require it to be especially strong, so crankshafts are hardened to improve durability; the two most common processes are induction hardening and nitriding. Engine oil is used to lubricate, cool, and clean the crankshaft journals and bearings. To control the destructive effects of both torsional and harmonic vibrations, diesel engines are equipped with vibration dampeners. Blocks must be designed to be as compact as possible, to reduce weight and size while maintaining strength and rigidity; common block designs include deep-skirt blocks, crossbolted blocks, ladder-frame blocks, and tunnel-bore blocks. There are several different configurations for engine block cylinders; which one is best depends on the vehicle’s application, production cost, and expected vehicle lifecycle. Replaceable cylinder sleeves enable a worn-out engine to be quickly rebuilt and restored to factory specifications. Bearings perform a number of important functions inside the engine, including reducing friction, supporting moving parts under load, and serving as replaceable wear surfaces. The characteristics important for engine bearings are compatibility, fatigue strength, conformability, and embeddability; a variety of construction techniques are used to

▶▶

▶▶ ▶▶

achieve different balances among the characteristics that will adapt to each engine’s unique operating conditions. The amount of oil clearance between the rotating shaft and the engine bearing is important to maintaining the hydraulic pressure of the oil wedge that cools and lubricates these parts. Bearings must not move within the bearing bore and must have good contact within the bore for proper heat transfer. Thrust bearings control end play in a crankshaft. Dual mass flywheels (DMFs) enable manufacturers to use lighter transmissions and avoid damage caused by torsional vibration.

Key Terms

cast-in-place sleeve A block construction technique that casts aluminum block material around an iron cylinder sleeve. cavitation erosion Pinholes produced in cylinder block walls, heads, and liner sleeves as a result of the collapse of tiny water vapor bubbles formed when coolant vaporizes on hot cylinder wall surfaces. compacted graphite iron (CGI) A material produced from powdered iron alloys, which is squeezed into molds at high pressures and then heated to bond the metal particles together; also known as sintered graphite. crossbolted block A variation of the deep-skirt block that uses additional horizontally placed bolts to connect the crankcase walls of the block to the main bearing caps; also known as a tiebolted or bolster-bolted block. deep-skirt block A block configuration with a crankcase wall that extends well below the crankshaft’s centerline. dry sleeve block A block designed with a bored or honed hole in the block that allows no coolant contact with the cylinder sleeve. dual mass flywheel (DMF) A two-piece flywheel design that incorporates specialized torsional dampening springs. fillet radius A round shaped machined intersection ground into the surface between the journal and the crankshaft cheek that strengthens the crankshaft and minimizes the possibility of a fracture. harmonic vibration A vibration that sends pressure waves moving back and forth along the crankshaft. induction hardening A heat treatment process that involves passing alternating electric current through coils of heavygauge wire surrounding the material to be hardened; through magnetic induction, heat is produced in the metal, which is then quenched with water to produce a hard, wear-resistant metal surface. ladder-frame block A block design that uses a ladder shaped component incorporating main bearing caps into a section that attaches to the crankcase and the oil pan.

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Wrap-Up

Nanoslide technology A spray welding (or plasma welding) technique developed by Mercedes-Benz to coat cylinder walls with a tool-grade hardness iron alloy. nitriding A crankshaft hardening technique that involves heating the shaft and introducing cyanide salt or liquid into the heating chamber with the crankshaft; slowly cooling or quenching the crank with these substances in its heated condition causes the nitrogen and carbon to molecularly bond with metal surfaces to create an extremely hard journal. parent bore block A block design that has holes cast and bored in the block for the cylinders with the pistons inserted directly into these holes; also known as a no-sleeve block. pendulum vibration absorber A dampener that provides torsional vibration control by producing forces that directly oppose and cancel the forces producing torsional vibration. primary balance Balance achieved when the crankshaft counterweights offset the weight of the piston and connecting rod assembly. scissor gear Two separate spring-loaded gears incorporated into a single unit to reduce gear rattle caused by torsional vibration. secondary balance Balance achieved when the movement of one piston counterbalances the movement of another, resulting in engines that run more smoothly. sputter bearing The latest technology in bearing overlay material, which deposits a metal overlay surface onto a bearing backing that is three times harder than conventional overlay. This complex process involves spray welding in a vacuum and is used to make a bearing that has the greatest ability to carry a load over any other bearing. torsional vibration The speeding up and slowing down of the crankshaft caused by alternating compression and power strokes in the engine cylinder. tunnel-bore block A block that has the main bearing bores and block formed into one solid structure for maximum block rigidity and strength. warp anchor A block design that has the cylinder head and the cylinder block bolted together by using tie bolts; sliding steel sleeves, which are locked in the block, accept the cylinder head bolt from one side and the tie bolt from the other. wet sleeve block A block designed with a number of large holes into which the cylinder sleeves are inserted; coolant has direct contact with the outside of the sleeve, and there is no cylinder bore structure.

Review Questions 1. What type of tapped threads is most commonly used in cylinder block castings? a. Fine. b. Coarse. c. Rolled. d. Metric and imperial. 2. Which of the following is the manufacturer’s purpose for core plugs? a. To clean sand from blocks after casting. b. To install block heaters.

115

c. To prevent freezing damage to cylinder blocks. d. To add inspection holes for internal block castings. 3. Which of the following is the primary reason for locating geartrains on the rear of the engine? a. To minimize engine noise. b. To ease of service. c. To make it easier to mount accessories such as power steering pumps and air compressors. d. To minimize engine torsional vibrations. 4. The presence of cavitation on a replaceable cylinder liner is most likely caused by ___________. a. liner vibration b. low cylinder pressures c. excessive cooling system pressure d. improperly conditioned coolant 5. Which of the following is the purpose of bolster bolts or tie bolts? a. To hold the main bearing caps down tightly. b. To reduce torsional vibrations. c. To increase engine block rigidity and strength. d. To secure the crankcase bedplate to the engine block. 6. Which of the following features will a ladder-frame engine block have? a. A block skirt extending past the main bearing caps. b. A steel plate bolted to the oil pan rail and main bearing caps. c. Main bearing caps integrated into a one-piece casting. d. A lightweight engine block. 7. Torsional vibration resulting in “jumped” accessory drive belts, excessive geartrain wear, and broken camshafts is commonly the result of which of the following defective components? a. Dual mass flywheel (DMF). b. Vibration dampener. c. Imbalanced crankshaft. d. Balance shaft. 8. The lightest, strongest, and quietest engine blocks are _________. a. made from aluminum b. parent bore blocks c. dry sleeve blocks d. made from compacted graphite iron (CGI) 9. V8 engines have cylinder banks arranged at which of the following angles to allow for even crankshaft rotation between firing impulses? a. 60°. b. 90°. c. 37.5°. d. 45°. 10. Crankshaft counterweights are used to minimize engine vibration caused by which of the following? a. The inertia of the piston/connecting rod assembly. b. Uneven firing impulses. c. Torsional vibration. d. Angular velocity changes of the crankshaft.

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Chapter 4 Cylinder Blocks and Crankshafts

ASE Technician A/Technician B–Style Questions 1. After discovering coolant is leaking into the engine oil, Technician A says the leak will rapidly accelerate cylinder wall wear. Technician B says the coolant leak is likely casue more damage to engine bearings first. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that polished cylinder walls will cause low power and more blowby. Technician B says cylinder walls with more than 90% remaining crosshatch have less than 50% remaining service life. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that Plastigauge that becomes wider indicates a smaller oil clearance between a crankshaft and main bearing. Technician B says the Plastigauge will become narrower when bearing oil clearances are smaller. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. While installing new rod bearing inserts, the shells protruded very slightly above the margin of the bearing cap. Technician A says this is normal. Technician B says that wrong bearings produce this result. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that rear geartrains are used to shorten the length of the engine block. Technician B says that rear geartrains reduce engine vibration. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

6. Technician A says that cast-in-place iron liners are commonly used in aluminum cylinder blocks. Technician B says that only closed-deck blocks will use cast iron liners. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. While examining a thick, heavy, flat steel plate bolted to the oil pan rails and main bearing caps, Technician A suggests it is used to stiffen the crankcase to reduce torsional twisting. Technician B says the plate is used to prevent oil from splashing up onto the cylinder walls and causing high oil consumption. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 8. Technician A says that crankshaft end play is controlled by the fillet radius of the crankshaft journals. Technician B says that the main bearings limit crankshaft end play. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says that a defective vibration dampener will cause the accessory drive belts of an engine to jump off the drive pulleys. Technician B says a defective vibration dampener will cause the crankshaft or even the camshaft to break. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that nitriding bearing journal surfaces produces the deepest amount of crankshaft hardening. Technician B says that induction hardening produces the hardest journal finish. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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CHAPTER 5

Cylinder Components Learning Objectives After reading this chapter, you will be able to: ■■

■■

5-01 Identify and describe the operating factors that affect how pistons are constructed. 5-02 Identify and explain the functions of major piston construction features. 5-03 Identify and explain the functions of piston ring construction features. 5-04 Identify and explain the purpose of piston ring material and shape according to ring function.

■■

■■ ■■

5-05 Identify and describe diagnostic techniques used for evaluating the condition of a piston ring–to–cylinder wall seal. 5-06 Identify and explain the function of piston pins. 5-07 Identify and explain the function of connecting rod construction features and mechanical stresses imposed on connecting rods.

You Are the Technician A 3500 series (class 2) truck arrives at your shop, and the driver complains that the engine lacks power and is running rough. During an inspection, you observe a knocking sound in the engine. There are many significant oil leaks from the gaskets and seals on the engine. In addition, a considerable amount of oil-laden blowby is being emitted from the crankcase tube connected to the turbocharger intake pipe, and oil has fouled the turbocharger compressor wheel. The engine oil level is low as well. When the throttle is snapped to accelerate the engine, visible black and gray smoke are observed at the tailpipe. Although the odometer reading is only 279,617 miles (450,000 km), the vehicle is close to 14 years old and is used primarily to pick up and deliver scrap metal in and around the city on a daily basis. Before you can prepare an estimate for the cost of repairs, you need to validate whether the base engine system is in satisfactory condition to justify any repairs or adjustments to the fuel system and other engine systems.

1. What techniques would you use to estimate the remaining service life of the engine, and why would you recommend them? 2. The accumulated distance on the odometer is relatively low for a diesel engine to be worn out and in need of an overhaul. What factors could contribute to premature wear of this truck’s engine? What observations could you make that would support your answer? 3. If the engine is overhauled or replaced and the vehicle is returned to service, what operating recommendations would you make to the driver to extend the life of the engine?

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Chapter 5 Cylinder Components

▶▶ Introduction Cylinder components are the engine components first used to convert combustion forces into mechanical power, which makes them a logical starting point to examine diesel engine construction. These components include pistons, piston rings, piston pins (also called wrist pins), piston pin bushings, and connecting rods (also called con-rods) (FIGURE 5-1). These components convert and transfer the forces of combustion to the crankshaft. They are the most highly stressed parts of the diesel engine. The flame temperature of diesel fuel is over 3,900°F (2,149°C), and cylinder pressures can momentarily reach 3,000 psi (207 bar) in turbocharged diesels compared to less than 1,000 psi (69 bar) in naturally aspirated spark-ignition engines (FIGURE 5-2). Because of these stresses, the design, construction, and operation of cylinder components are critically important to engine performance and durability. In addition, cylinder components, together with other engine systems, have features needed to reach emissions targets. Emission legislation imposes new design quality on cylinder components through durability requirements. Legislated standards for the engine durability require noxious emissions to remain below maximum thresholds through the expected useful life of an engine. To keep emissions low throughout an engine’s anticipated lifecycle, cylinder components are now Piston constructed to last longer than ever. Tier 3 emission standards, which are Rings similar to Euro 6 and California LEV-III, require light- and medium-duty Piston Pin diesels to have a useful life of 150,000 miles (241,400 km) or 15 years (TABLE 5-1). A shorter useful life of 120,000 miles (193,000 km) or 10 to Connecting 11 years is permitted only if vehicles certify to lower initial limits for NOx Rod and hydrocarbon emissions. Cylinder components in contemporary diesels have undergone numerous refinements to help meet higher standards for power density, performance, life expectancy, and low emission output. Major engine repairs involve replacing or determining the serviceability (remainOil Cooler Nozzle ing service life) of these parts, so technicians need to familiarize themselves with the construction, operating principles, and failure modes of these parts. Engine failures often involve damage to these components, so understanding failure modes is necessary in order to correct the root causes of damage to FIGURE 5-1 Cylinder components include the piston, these components. piston rings, piston pin, and connecting rod. °F °C 3,000 psi + (207 bar +)

+3,900°F + (2,149°C +)

FIGURE 5-2 High combustion

pressures and temperatures require pistons to be capable of withstanding high thermal loads and cylinder pressures.

TABLE 5-1 Increasing Emission Durability Standards Requires More Durable Engine Components Emission Standard

Distance

Time

Maximum Limit

Tier 3 (2017–2025)

150,000 miles (241,400 km)

15 years

NMOG + NOx

Tier 2 bin 5 (2007–up to 2019)

120,000 miles (193,000 km)

11 years

FTP* Standards

EPA** (LEV-III) (2015–2025)

120,000 miles (193,000 km)

11 years

NMOG + NOx × 0.85

* FTP: Federal Test Protocol (A standardized emission testing procedure) **EPA: Environmental Protection Agency Source: U.S. EPA

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Piston Classification and Design

119

FIGURE 5-3 The piston crowns of DI

and IDI pistons.

▶▶ Piston

Classification and Design

Since pistons are relatively uniform in their function, their classification often follows 5-01 Identify and describe the operating factors that affect how the construction details of the component. For example, the shape, materials, or conpistons are constructed. struction techniques help differentiate the different types of pistons. In direct-injection (DI) diesel engines, the combustion chamber is formed in the piston. Compression Ring Indirect-injection (IDI) engines shape the piston crown to direct the Intermediate Ring flow of burning combustion gases from the pre-chamber to the main Oil Control Ring chamber (FIGURE 5-3). Tremendous heat, pressure, and inertia forces are sustained by pistons Piston Crown as they capture combustion energy and transmit it to the other reciproPiston Skirt cating and rotating parts of the engine (FIGURE 5-4). In order to operate Piston Pin reliably for thousands of hours under extreme conditions critical construc- Snap Ring tion features are built into piston design. Piston design is a major factor in determining a variety of engine characteristics such as power output, emission characteristics, maximum speed, oil consumption, and durability. Bushing Before examining any specific features of each type of piston, it’s helpful to identify the parts common to all pistons. Pistons have three main sections (FIGURE 5-5): 1. the crown, or top of the piston, which is subjected to cylinder pressure and heat 2. the ring belt, which is where piston grooves are located 3. the skirt, which is the surface that slides directly against the cylinder wall to stabilize piston movement. In the uppermost part of the skirt is the piston pin. Additional piston material reinforces the area around the pin; this material is called the pin boss (FIGURE 5-6). Steel struts are often cast into the pin boss for improved strength.

Connecting Rod Cap

Bearing Shells

Bolts

FIGURE 5-4 Identification of various piston and connected

cylinder components.

Crown

Ring Belt

Skirt

FIGURE 5-5 The main parts of a piston.

FIGURE 5-6 Piston nomenclature. The connecting rod and piston are forged.

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Chapter 5 Cylinder Components

Types of Pistons Four basic piston designs are used in today’s diesel engines: Slipper Skirt

1. trunk 2. slipper skirt 3. articulated or low crevice volume (LCV) 4. Monosteel.

Rotation of Crankshaft Counterweight

Slipper Skirt Pistons

In the design of a slipper skirt piston, a portion of the skirt is removed on both non-thrust sides of the piston. This provides clearance for the turning radius of the crankshaft counterweights when the piston approaches bottom dead center (BDC) (FIGURE 5-7). Removing skirt material enables slipper skirt pistons to use a shorter connecting rod, the smaller combined length of the rod FIGURE 5-7 This piston uses a slipper-skirt-piston and piston assembly enables vertically shorter, lighter, more compact engine design. blocks to be manufactured around it. This is important when the profile of the engine must be kept small to fit in a smaller vehicle. More compact engine size translates to lighter front end designs for better vehicle dynamics. When engine mass and inertia are lighter, the vehicle will depend less on heavier front end components needed to support engine weight, the front end will dive less during braking, and become more responsive turning when engines are lighter. Slipper skirt pistons are essential in smaller V-block diesel engines, where compact engine design is vital.

Trunk-Type Pistons Trunk-type pistons describe a piston construction that has a full skirt (FIGURE 5-8). The fullskirt design is stronger, resisting side thrust loads better, so that cylinders can support higher cylinder pressures. Trunk-type pistons are used in more rugged applications, such as in-linesixes with high horsepower (kilowatt) output. Side thrust is the force of a piston against the side of a cylinder wall produced by the angle of the connecting rod with the piston, which pushes the piston sideways when cylinder pressures are high. Cylinder pressure during power stroke produces major side thrust, while compression pressure produces minor side thrust (FIGURE 5-9).

Articulating Pistons FIGURE 5-8 A trunk-type piston having

a full piston skirt.

Another innovation in piston technology is the use of articulating pistons (FIGURE 5-10). These pistons are used in many medium- and heavy-duty diesel engines, but they are not yet used in light-duty diesels. Articulated pistons are constructed by using two pieces: a

Center Line

Pin Offset

Minor Side Thrust

Compression Stroke

Major Side Thrust

Before TDC

Power Stroke

FIGURE 5-9 Major and minor thrust forces.

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Piston Classification and Design

121

Crevice Volume

FIGURE 5-10 An articulating piston

is made of two pieces: a steel crown connected to an aluminum skirt by using a piston pin.

separate aluminum skirt connected to an alloy steel crown by using a piston pin. The use of steel as a construction material helps these pistons resist damage from high temperatures while supporting greater cylinder pressures. Steel piston crowns also reduce the volume of the area above the top ring and between the piston and cylinder wall (FIGURE 5-11). This region between the piston crown and cylinder wall above the top compression ring is called the crevice volume. The strength of a steel crown enables the top ring to be placed closer to the crown without causing a fracture between the ring groove and crown. Pistons with this feature are referred to as LCV or Low Crevice Volume pistons. Manufacturers’ studies have found that this change alone improves fuel economy by as much as 5%. Eliminating the dead air of the crevice volume area lowers hydrocarbon (HC) exhaust emissions by as much as 50%. Trunk pistons are also made with reduced crevice volume, but aluminum pistons cannot reduce the crevice volume to the same extent as pistons with steel crowns can.

FIGURE 5-11 Crevice volume refers

to the area between the piston and cylinder wall above the top piston compression ring.

Monosteel Pistons Monosteel pistons—also known as Ecotherm or Monotherm pistons—are the newest innovation in diesel piston design (FIGURE 5-12) now used by even smallbore light-duty diesels. The most noticeable feature of these one-piece alloy steel design is its use of less material between the piston crown and piston pin (FIGURE 5-13). Shortening the piston achieves nearly a 50% reduction in the distance between the centerline of the pin bore and the top of the piston—a dimension referred to as the compression height. Thinner skirt walls, a large cooling oil passageway, and the lower compression height help make these pistons comparable in weight to aluminum pistons used in similar applications. The smaller compression height has the potential to enable the use of shorter connecting rods and reduced block height to lower engine size and weight (FIGURE 5-14).

FIGURE 5-12 A Monosteel piston.

Aluminum Steel 30% Less Compression Height

FIGURE 5-13 The use of steel pistons can reduce a piston’s compression height.

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Chapter 5 Cylinder Components Steel Alloy Crown

Aluminum Skirt Single Piece

MONOTHERM

Trunk

Articulating

FIGURE 5-14 Comparing aluminum- and steel-type pistons.

In comparison to aluminum, which expands at a rate of four to seven times faster than steel, Monosteel or articulated pistons can operate with tighter operating clearances. Operating clearances are required to allow the piston to expand when heated and allow oil to lubricate moving parts. With reduced operating clearances, both piston slap noise at cold start-up and low-load conditions are minimized. Using steel as a piston construction material essentially reduces blowby and oil consumption because of the tighter operating clearances. Using steel pistons in a diesel engine has decreased fuel consumption by as much as 2% in test conditions. Much of the improvement in efficiency is achieved through a reduction in friction and tighter operating clearances. Low expansion ratios and a smaller piston skirt area due to the use of stronger steel provides 1% higher power output and thus 1% lower fuel consumption. This improvement also decreases turbocharger speed by 1,000 rpm in some engines, leading to improved turbocharger durability. More important are the benefits of increased durability and the ability to withstand higher temperatures and pressures than aluminum pistons. Current maximum cylinder pressures for these pistons is 3,770 psi (260 bar). This advantage alone increases power density from an engine, which means more power can be produced from each cubic inch (or cubic cm) of engine displacement. Higher piston temperatures reduce carbon formation and improve engine thermal efficiency. The following are some of the other advantages of using steel pistons: ■■

■■

Alloyed steel construction means that tear apart forces during catastrophic failure are lower, leading to less engine destruction. No piston pin bushing is required by the steel construction. Instead, the wrist pin bore is coated in phosphate to reduce friction. A phosphate coating reduces the possibility of corrosion in exhaust gas recirculation (EGR) engines having more acidic oil.

▶▶ Piston 5-02 Identify and explain the functions of major piston construction features.

Construction

A piston may be made through a casting process or a forging process. In a casting process, molten metal is injected into a mold from the bottom up. This minimizes the likelihood of trapped air pockets forming in the casting. Steel reinforcing struts are placed in the pin boss area, and hardened nickel alloy inserts for ring grooves are also fixed in aluminum castings during this process. When cooled, the piston features are machined to dimensions and to accept rings and wrist pins. Pistons are also made by using a forging process to produce a much stronger but heavier type of piston. When forged, a metal billet is heated to a plastic-like state. The part is placed into a mold with two halves, where it is squeezed by high pressure into a finished shape. Final machining is done afterward, including trimming away the flashing or excess metal squeezed out between the halves of the mold, called parting lines (FIGURE 5-15). This process of forging compacts the metal structure more tightly, forming a denser, heavier piston and realigns the grain structure of the metal. Forged pistons are able to withstand

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Piston Construction

FIGURE 5-15 Parting lines on a forged piston.

123

FIGURE 5-16 The finely machined lines on this piston skirt are used to

retain oil for lubrication.

higher cylinder pressures and temperatures than cast pistons can. The denser structure will conduct heat away faster than a cast piston will, reducing its operating temperature by 20% compared to a cast piston. However, a disadvantage to forged pistons is that they have a greater expansion coefficient than cast pistons, thus requiring larger running clearances in cold engines. Pistons may have the crown and skirt manufactured in two separate processes. The skirt may be forged and the crown cast, but both are bonded together by welding or by simply casting the crown onto the forged skirt. Subsequent to shaping, skirts made of aluminum pistons are anodized to resist wear. Anodizing is an electrochemical process in which aluminum is combined with oxygen to create a tough aluminum oxide finish. A micro-finish etching or machining of fine lines is commonly added to the piston skirt to retain oil (FIGURE 5-16). Oil trapped on the skirt’s micro-finish is used to help lubricate the piston.

Piston Cooling High cylinder temperatures and pressures found in diesel engines FIGURE 5-17 A cracked piston crown, where the crack was caused exert high heat loads on pistons. To prevent piston failure, several by inadequate cooling during the valve overlap period. This engine’s air mechanisms are used to transfer heat away from the pistons. First, filter was plugged. piston rings can transfer some heat to the cylinder walls through the piston rings and skirt. As much as one-third of the heat can be removed this way. Heat is also removed during the valve overlap period. During the valve overlap period, when ▶▶TECHNICIAN TIP the intake and exhaust valves are both open at the end of exhaust stroke and the beginning Whenever cylinder components are of intake stroke, fresh, cool air mass flowing between the intake and out the exhaust valve removed, always remove the cooling can cool the piston crown (FIGURE 5-17). Most of the cooling for diesel engine pistons, nozzle first. Failure to first remove these however, is accomplished through spraying the pistons with oil under the crown surface nozzles may result in damage or mis(FIGURE 5-18). Passageways formed in the pistons will also help transfer heat to the oil alignment during service, resulting in an (FIGURE 5-19). Heat absorbed by the oil is transferred to the cooling system through the oil improperly cooled piston. Incorrectly cooler. This piston cooling feature helps explain why during engine assembly and service, aimed nozzles and plugged or restricted the aim of the cooling nozzles to the piston is critical to ensure adequate heat transfer and nozzles will result in an overheated piston. prevent piston failure.

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Chapter 5 Cylinder Components

Oil Cooling Gallery

Oil Spray from Cooling Nozzle

FIGURE 5-18 Oil cooler nozzles spray lubricating oil to the underside

FIGURE 5-19 Passageways inside the piston allow oil

of the piston crown, removing excess heat.

to circulate and remove heat from the piston.

Hole Burned by Continuous Fueling

Rounded, Burned Crown

FIGURE 5-20 This aluminum piston crown melted due

to a failure in a common rail injector. After the crown overheated, cylinder pressure pushed a hole through the piston.

Aluminum Piston Materials Due to the high speed of a reciprocating engine and the tremendous inertial forces of the piston at the end of each stroke, aluminum alloys are the material of choice for most light-duty diesel pistons. Lower inertial forces translate into less stress on other engine components, such as connecting rods and crankshafts, which extends engine life. Using aluminum alloyed pistons minimizes engine vibration and harshness. In addition to being one-third lighter than steel or iron, lightweight aluminum conducts heat about four times faster than iron. This means more heat will be transferred from the piston to the cylinder wall and engine oil. Aluminum is relatively easy to machine as well. However, when aluminum is used as a piston material, it comes with a number of disadvantages. 1. Aluminum has a low melting point. Since aluminum has a melting temperature of about 1,250°F (675°C) and the flame temperature of diesel fuel is 3,900°F (2,149°C), aluminum can potentially melt during extreme engine operating conditions. To overcome this, aluminum is alloyed with about 10% of other materials, such as silicon, copper, zinc, and chromium. Silicon, which makes up quartz or beach sand, gives the metal higher temperature resistance and minimizes thermal expansion. A hypereutectic piston has 16–20% silicon content, whereas other pistons have 8–11% (the maximum saturation point, or eutectic point, of aluminum is 12% silicon) (FIGURES 5-20, 5-21, and 5-22).

FIGURE 5-21 This melted aluminum piston has

a grainy texture along the edges of the piston crown, demonstrating the high silicon content. The overheating may have been caused by overfueling, advanced injection timing, or inadequate piston cooling.

FIGURE 5-22 The underside of a diesel piston crown will show

evidence of baked oil from high crown temperatures. The right pistons discoloration is at the limit of normal.

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Piston Construction

125

FIGURE 5-23 Location of a Ni-Resist insert in a Ford five-cylinder 3.2 L diesel.

2. Aluminum is soft and has poor wear characteristics. This problem can be partially overcome by alloying aluminum. One of the areas of the piston that experience the greatest wear is the top ring groove. The high combustion temperatures, combined with ring movement in the top ring groove, can cause rapid wear in this area. To prevent this, a band of special stainless steel alloy is cast into the top ring land or even both ring lands. The band is machined with a groove to accept the piston ring. This band is commonly known as a Ni-Resist insert (FIGURE 5-23). A Ni-Resist insert further reinforces the top ring piston land, which will prevent the top of the weaker aluminum of the ring land from breaking away. Installing this land comes with the disadvantage of adding complexity and cost to the manufacturing process. 3. Aluminum has a high expansion coefficient. Aluminum pistons have a high expansion coefficient, which means they will expand four to seven times more than ferrous metal when heated. Aluminum alloy pistons will require large operating clearances between the piston and cylinder wall to prevent the piston from seizing after the piston warms up to operating temperature. If this expansion is not controlled, the piston will swell and score the cylinder and piston or even seize in the cylinder. Using excessive clearances is not an alternative, since this will result in piston slap or noise when the engine is cold. This noise is the result of the piston running loosely within the cylinder. Collapsed skirts, poor sealing, and oil control are some other undesirable results of running excessive clearances.

▶▶TECHNICIAN TIP Piston slap refers to a noise pistons make that can be heard in an engine during warm-up and light-load operation, when the piston has not fully expanded in the cylinder bore.The noise should disappear when the piston is warmed up.

▶▶TECHNICIAN TIP Aluminum alloy pistons are common in light-duty diesels and operate with larger clearances between the cylinder wall when the engine is cold. It is important to warm up the pistons before heavily loading an engine. This can be accomplished through operating the engine at high idle for a couple of minutes. It is important to engage light-load operation for the first few minutes to prevent piston damage from collapsed piston skirts.

Cam Ground Pistons A number of strategies are used to minimize the expansion problems of pistons. One method is to alloy the piston with silicon. Aluminum pistons are alloyed with approximately 20% silicone to add heat resistance and minimize expansion. At this high percentage of alloying, aluminum pistons are occasionally referred to as eutectic pistons. However, the method that is used almost universally to control expansion is to manufacture the piston in an elliptical or oval shape. Elliptically shaped pistons permit expansion when heated yet

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allow for minimal operating clearances to reduce noise and slap. When pistons are manufactured into an elliptical or oval shape, the expansion can be controlled so that the piston can expand to a symmetrically round shape. This piston configuration is also commonly known as a cam ground piston (FIGURE 5-24). Most of this expansion takes place in the dimensions where the most metal and heat are. This means the crown will expand the greatest amount when heated. Because the wrist pin bore is reinforced, it will expand to a large extent as well. Areas that expand the least are the botCylinder Wall tom of the skirt and thinner skirt areas around located at 90° from the Normal Cool Expanded Hot wrist pin. This area corresponds to the major and minor thrust surfaces FIGURE 5-24 Pistons are machined not to be perfectly round of the piston. Knowing this helps explain why measuring the piston but instead to be elliptically shaped, so that they can expand diameter should be done at 90° opposite to the piston pin bore since it is while reducing the looseness of the piston in the cylinder. the widest cross section of the piston. Aluminum pistons also possess a barrel or taper shape. The crown and ring belt parts of the piston are significantly smaller than the bottom circumference. A slight bulge in the middle section is built in to minimize noise and slap and to enhance sealing capabilities. Since the cam ground piston is irregularly shaped, rings and not the piston are responsible for sealing the clearance between the cylinder walls when the engine is operated cold. This explains why piston rings on aluminum pistons will generally be thicker and require greater ring tension to ensure a gas-tight seal. Piston Cross Section

Aluminum Pistons and Emissions

▶▶TECHNICIAN TIP A diesel engine will often run rough, accelerate slowly, and lack power if it has idled for extended periods of time. This condition is caused by cylinder glazing. Even as little as 10 or so minutes of idle can produce enough glazing to cause a significant change in engine performance. Tow trucks, ambulances, and emergency vehicles, which spend a lot of time idling, are especially prone to cylinder glazing.To prevent this, excessive engine idle should be avoided. When it does happen, operating the engine under load at highway speed will clear the glaze from the cylinder walls and restore normal engine performance.

Aluminum trunk-type pistons were satisfactory for many years and continue to be used today in many engines. However, other disadvantages to aluminum pistons have arisen as manufacturers have strived to reduce engine emissions and increase engine longevity and horsepower (kilowatts) all at the same time. Because aluminum pistons cannot withstand the high cylinder pressures and temperatures produced by high engine power output, new materials have been sought to replace it. Ceramic-coated pistons and ceramic alloy crowns are two such replacement materials. These pistons can not only withstand greater thermal loading but help reduce heat rejection loss from the combustion chamber. Reducing how much heat transfers to the coolant or oil means more fuel energy is converted into mechanical energy (FIGURE 5-25). Of course, new trunk-type pistons made entirely of steel are being introduced for a number of reasons.

Engine Damage at Idle It is important to know that idling a diesel engines for prolonged periods of time shortens engine life and damages cylinder components. Idling damages cylinder components in several ways. 1. Slobber production. Combustion and exhaust temperatures at idle in diesels are relatively “cold.” This happens because excess air combines with small quantities of fuel injected at idle to produce low cylinder temperatures and pressures. Pistons do not expand properly when cold, resulting in poor cylinder sealing. When pistons are insufficiently sealed, Steel Alloy Crown

Aluminum Skirt Single Piece

MONOTHERM

Trunk

Articulating

FIGURE 5-25 Comparing an aluminum piston with Monosteel and articulated. Note the distance between

the top ring and the crown. Aluminum pistons have larger clearance volume.

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Piston Construction

compression pressure slips past the rings but allows more lubricating oil to enter the combustion chamber. Unburned oil, fuel, and soot are produced from low-temperature, low-pressure combustion, which when mixed forms engine “slobber.” This slobber will accumulate in the exhaust system and stick around the exhaust valves, around the piston rings, and in the ring belt area. If this slobber gets around the piston rings and hardens, the rings will stick in their grooves and become almost ineffective. Slobber is one of the most common causes of premature EGR valve failure because it will contaminate or “gum up” EGR valves, causing them to stick. DI engines are especially prone to slobbering because of their high thermal efficiency and low heat-transfer rate to engine coolant. Slobber is a substance that must be purged from the cylinder during combustion or prevented from forming; otherwise, slobber residue can accumulate and cause engine damage. During loaded engine operation, heat hardens the deposits into very abrasive carbonized crystals that eventually score the cylinder walls (FIGURE 5-26). 2. Excessive soot loading in engine oil due to idling and prolonged low engine load operation. Soot loading accumulating in engine oil is the result of poor combustion quality when the engine is at idle. Soot clings to the cylinder wall and is scrapped down into the crankcase to contaminate lubrication oil. A large amount of soot loading in the oil shortens the maintenance intervals between oil changes and requires that oil filters need to be changed more frequently. Soot is also an abrasive compound that will cause premature engine wear if allowed to reach more than 5% contaminant loading. 3. Cylinder glazing. Oil fills the crosshatch finish of the cylinder walls during idle and is baked onto the cylinder wall. Piston and ring travel smoothen the baked oil into a mirrorlike finish on the cylinder wall, which is then unable to retain oil. This process, called glazing, prevents the piston rings from effectively sealing leading to a loss of compression through blowby. When an engine is rebuilt, it is especially critical to prevent the cylinder walls from becoming glazed, by placing the engine under load soon after the engine starts. The deep cylinder wall crosshatch and poorly seated new rings will allow an engine to quickly glaze to the point where it is very hard to start and has low power. Shortly after starting, applying a heavy load and running the engine for at least 45 minutes to an hour helps seat the rings into the cylinder wall. This process, called break-in, uses high cylinder pressure to push the rings into the cylinder wall, where friction will smooth both ring face and cylinder wall surfaces to ensure that the rings better conform to the cylinder wall. Rapid break-in after rebuilding will ensure reliable and powerful new engine performance (FIGURES 5-27, 5-28, and 5-29).

127

FIGURE 5-26 This piston has operated

for prolonged periods at idle, leading to excessive carbonizing in the ring belt area.

▶▶TECHNICIAN TIP Engines that operate under severe conditions, such as prolonged idle conditions in cold weather, require shorter service intervals between oil changes. Under these conditions, engine lubricating oil will contaminate faster, with soot produced by poor quality combustion. Because soot contaminates the lubricating oil for diesel engines, that oil will blacken faster than gasoline engine lubricating oil.

▶▶TECHNICIAN TIP

FIGURE 5-27 Crosshatch is observed by

the fine intersecting lines on the cylinder walls. Crosshatching provides a cylinder wall finish, which retains engine oil needed for piston lubrication and to promote a gastight piston ring seal.

FIGURE 5-28 A ball-type hone can be used to apply

new crosshatch finish to a cylinder.

To avoid engine damage, diesel engines should never be operated at idle speed with no load for prolonged periods. If an engine must be idled, it is best to operate it at high idle. Manufacturers are using engine calibrations that increase engine speed and change injection timing if oil and coolant temperatures are low. Similarly, many late-model diesels have limited engine speed when cold. This ensures cold, soot thickened oil is flowing to critical engine parts and that the pistons are adequately warmed before fully loading the engine. Even automatic transmissions are now adding supplemental loads on the engine to speed up warm-up. Adding cetane booster to fuel in cold weather regions can help reduce the slobber production that contaminates turbocharger actuators and EGR valves.

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FIGURE 5-29 Honing stones used to apply a crosshatch finish

FIGURE 5-30 Inspection points on pistons.

to a cylinder wall.

Piston Cleaning and Inspection

▶▶TECHNICIAN TIP The small clearance volume in diesel engines makes the engine vulnerable to hydrostatic locks. This occurs when liquids such as oil, coolant, or fuel enter the cylinder during servicing or through leaking head gaskets or water ingestion through the intake or porous cylinder walls. A bent connecting rod or locked engine will be the result of hydrostatic locks. In severe situations, cranking the engine can result in broken cylinder walls and bent valves. If a hydrostatic lock is suspected, barring the engine over in the opposite direction of rotation will usually allow the engine to rotate 360°. During normal rotation, the engine will stop during the compression stroke of the cylinder that contains the liquid. Liquids can be vacuumed through injector holes in the cylinder head. Slightly holding an exhaust valve open while barring the engine over will also clear a lock.

Pistons have a micro-finish on the skirt that allows them to retain oil to improve wear characteristics. Some pistons have anodized skirts. Anodizing is a hardening process electrically imparted to the aluminum. Anodizing combines oxygen with the aluminum to form aluminum oxide, a very tough, corrosion-resistant material. Therefore, it is important to remember that this finish should not be damaged during servicing. The temptation to clean pistons with abrasive cleaners, wire wheels, and other things should be resisted. Soaking aluminum pistons in a mild aluminum-compatible cleaner followed by steam cleaning the pistons is usually the best acceptable method for cleaning. Hard carbon deposits on the crown may be scraped off by using a plastic scraper. Visually inspect pistons for evidence of cracking, overheating, and wear (FIGURE 5-30). The presence of vertical or horizontal cracks in the piston bowl or wrist pin bore is important when performing failure analysis (FIGURE 5-31). Horizontal cracks usually indicate a bonding problem between the piston skirt and crown. Vertical cracks often indicate high cylinder temperatures. Worn pistons or pistons with a collapsed skirt, which is a deformed skirt caused by lugging a cold engine result in engine noise, high oil consumption, excessive blowby, and power loss. To inspect whether a piston is reusable or damaged or to inspect the fit of a new AQ1 piston in a cylinder, follow the steps in SKILL DRILL 5-1. Clearances between the cylinder wall and piston can be checked by using a long feeler blade between the piston and cylinder wall at 90° to the pin bore. A worn piston or cylinder wall or a collapsed skirt is indicated when clearances exceed 0.004" (0.1016 mm). Crack Progression Pin Bore Crack

FIGURE 5-31 Piston cracks between

the piston pin bore and the crown indicate abnormally sharp spikes in cylinder pressure caused by conditions such as the improper use of ether as a starting aid or hydrostatic locks.

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Piston Ring Construction and Functions

129

SKILL DRILL 5-1 Measuring Piston to Wall Clearance 1. Before installing piston rings, insert the inverted piston into a clean and properly honed or prepped cylinder. 2. Place a feeler blade between the piston and the cylinder wall at 90° to the piston pin bore. Using the largest feeler blade that will slide with a reasonable amount of drag, measure the clearance between the skirt and cylinder wall. 3. Compare the thickness of the feeler blade with the specifications the original equipment manufacturer (OEM). Generally, the clearance is ideally 0.0035" (0.0889 mm) between the piston and cylinder wall. This clearance allows for just an adequate film of lubricating oil.

Micrometer

Piston Ring Gauge

Piston FIGURE 5-32 Measuring ring groove wear with a go/no-go

FIGURE 5-33 Gauges with the same profile as a ring

gauge.

are used to inspect piston ring grooves for wear.

Wear in the pistons ring groove dimensions are typically checked with specialized OEM gauge pins or go/no-go gauges (FIGURE 5-32 and FIGURE 5-33). Worn ring grooves will cause excessive oil consumption.

▶▶ Piston

Ring Construction and Functions

The piston rings are some of most critical components to ensuring engine durability and efficient operation. Rings wear faster than other internal engine parts, thus limiting engine life. Worn rings cause a loss of compression, resulting in low power and excessive blowby. Piston rings perform the following functions: 1. They form a gas-tight seal between the piston and the cylinder wall. The gas-tight seal is formed in combination with an oil film on the cylinder walls, which enhances the sealing between the ring and cylinder wall. Ensuring that the cylinder has a gas-tight seal is necessary to provide adequate compression pressure and to minimize power-robbing blowby, which also leads to oil contamination and excessive crankcase emissions (FIGURE 5-34). 2. Assist in cooling the piston by transferring heat to the cylinder wall.

5-03 Identify and explain the functions of piston ring construction features.

FIGURE 5-34 Identifying piston rings.

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Chapter 5 Cylinder Components

Cylinder Pressure

Piston Ring Blowby Compression Rings Road Draft Tube

Blowby Gas

Crankcase Oil

FIGURE 5-35 Piston rings provide a gas-tight seal in the cylinder to minimize loss of compression and blowby.

3. They apply a film of lubrication oil on the cylinder wall of the engine. The film must provide adequate lubrication properties, yet not be excessive, because excessive lubrication will cause high oil consumption and excessive emissions. 4. They form a compatible, replaceable wear surface with the cylinder wall. Compatibility refers to the ring’s ability to operate with as little friction as possible. The rings must perform these jobs under extreme temperatures and for the entire life of the engine. In fact, one of the primary determinants of engine life is the ring’s ability to do the above jobs efficiently. Ensuring long ring life is a matter of ring design, correct service practices, and proper engine operation (FIGURE 5-35).

▶▶ Classifying 5-04 Identify and explain the purpose of piston ring material and shape according to ring function.

Piston Rings

There are three types of piston rings found in most engines: 1. compression 2. oil control 3. intermediate (a combination of compression and oil control).

Exhaust Gas Recirculation Engines and Ring Longevity Refinements to ring design, cylinder bore finishing techniques, and engine oils have helped produce engines that can operate for thousands of hours and for more than a million miles or kilometers of use before rings need replacement. Ring design and the performance of engine oil are even more critical with the use of abrasive EGR-equipped engines. Higher cylinder temperatures and pressures combined with reintroducing soot and corrosive exhaust gases found in EGR engines can affect ring longevity. For this reason, ring

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Classifying Piston Rings

131

technology, which was relatively stable, has changed to compensate for these new wear factors. New materials and manufacturing techniques have produced some of the most sophisticated and durable rings ever made (FIGURE 5-36).

Compression Rings Compression rings are located nearest the piston crown (FIGURE 5-37). Their primary function is to prevent gases from leaking past the piston during compression and power stroke. When gas leaks past these rings, referred to as blowby, it robs power since it reduces compression pressures and may even allow combustion gases to leak into the crankcase.

Gas Sealing Compression rings seal in a number of ways. One way is through the static force of ring tension against the cylinder wall. Since rings are manufactured, having a larger relaxed diameter than the diameter of the cylinder they are to be installed in, the ring’s spring tension seals by expanding against the cylinder wall. In older rings, static FIGURE 5-36 Using EGR reintroduces exhaust gases into the ring force may easily be over 20 lb (7.465 kg). However, in an effort cylinders, which contain soot. Soot is an abrasive compound that to minimize friction losses in an engine, high ring tension is not a can accelerate engine wear, including cylinder wall and ring wear. desirable feature in a ring. Thinner rings can in fact seal better using a knife-edge like sealing. This means more pressure is directed against a small surface area. Today, ring tension may be as low as 4–8 lb (1.5–3 kg) of ring force. Low-tension rings can significantly improve fuel economy by decreasing internal friction and cylinder wear. Another way compression rings seal is through the use of gas pressure during compression and power stroke to push the ring out harder against the cylinder wall (FIGURE 5-38). To accomplish this, the thickness of the ring is made to be slightly smaller than the height of the ring groove. With this arrangement, gas pressure is able to get behind the ring and push the ring out against the cylinder wall harder than ring tension can. Gas pressure also pushes down on the ring against the bottom of the groove for better sealing. This means the higher the cylinder pressure, the greater the ring’s ability to seal. During power stroke, the compression and intermediate ring seals more effectively. This explains why most blowby Cylinder Pressure occurs during compression stroke. Piston speed and size are often factors in determining how many compression rings are used. Slower speed engines require more rings to minimize the loss of compression pressure. Compression rings, like all piston rings, are split for installation and to accommodate thermal expansion. Gases can escape through the ring end gap, so two compression rings are sometimes used on pistons. Compression Rings

Piston Ring End Gap A minimum ring end gap is critical to prevent losing compression and excessive engine blowby. Too small of a ring gap will cause the ends to butt and prevent expansion. Excessive gap will obviously cause a loss of compression and excessive blowby. A general specification for ring end gap is 0.003–0.004" per inch (0.0762–0.1016 mm per 2.54 cm) of cylinder bore. One of the primary reasons for ring breakage in worn engines is flexing of the ring as it moves from a narrow diameter to a larger diameter and back again, repeatedly. For every

Blowby Gas

Barrel Face Compression Ring Witness Line Lower Edge Wear of Taper Faced Intermediate Ring

FIGURE 5-38 Gas pressure above the

Oil Scraper Ring with Coiled Expander

FIGURE 5-37 Identification of

common ring configuration.

compression and intermediate ring forces the ring out against the cylinder wall. Making the ring groove slightly larger than the ring and notching the backside of the ring enhances this effect.

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Chapter 5 Cylinder Components Heavy Ring Wear

Plasma Face

Material Flaking

New

Worn

Little Material Remaining

FIGURE 5-39 This ring is

severely worn from dirt entering the engine. Wear like this is referred to as engine “dust out.”

100% Worn

Original Ring Thickness

0.001" of cylinder taper, the ring end gap will expand slightly more than 0.003". This flexing will eventually fatigue the ring and cause it to break. Maximum cylinder taper is limited in most engines to 0.001" per inch of bore. No more than 0.005" of taper is generally allowed in a cylinder. A dial bore gauge is generally used to measure this taper. An alternative method is to measure the ring end gap with the ring at several locations in the cylinder: near the top, middle, and bottom.

Compression Ring Construction and Function Cast iron rings were used for ring material in early automotive days. Iron is an ideal ring material because of its ability to break in easily and conform to cylinder irregularities and because of its high heat resistance. Today, cast iron rings may be only occasionally used when rebuilding gasoline engines and never used for diesel engines. Ductile iron or steel rings are the most common ring materials used today, with special coatings applied to the ring face to improve hardness or oil retention. Turbocharged diesel engines with high temperatures and cylinder pressures require more durable ring materials. Malleable iron was often used as the ring base material since it will bend and not fracture like cast iron rings. Chrome plating was used on the ring face to improve wear and heat resistance. This material started to be used in World War II to prevent engines in desert tanks from dusting out due to ingesting desert sand. During dust out, sand enters the cylinder and causes rapid abrasive wear (FIGURE 5-39). Chrome has one disadvantage, however: its smooth face will not retain oil for lubrications. The consequence is higher cylinder wall wear in the ring turnaround area since upper cylinder wall lubricant is nonexistent above the intermediate ring (FIGURE 5-40). During the 1970s, ring manufacturers found that a coating of molybdenum on the ring improves wear characteristics. Wear resistance is improved because “moly”-faced rings have a porous finish. This porosity keeps oil on the ring face so that oil can be carried on the ring face, thus reducing upper cylinder wall wear (FIGURE 5-41). A combination of chrome and molybdenum was used for decades for compression rings. FIGURE 5-40 Most cylinder wear takes place at the top and

tapers toward the bottom. A lack of lubrication oil above the oil control and intermediate ring is the primary reason for most wear taking place in the ring turnaround area.

Steel Compression Rings A much better material for compression rings than cast iron and ductile iron is steel. This material can be more than three times as strong as ductile iron. Since steel is also the best material for withstanding the pressure

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Classifying Piston Rings

TABLE 5-2 Comparing Piston Ring Alloys

Chrome Ring

Material Hardness

Tensile Strength

Gray cast iron

20,000 psi (140 mPa) minimum

Ductile iron

40,000 psi (275 mPa) to over 90,000 psi (620 mPa)

Steel

200—2,100 mPa 29,008—304,579 psi

and temperature loads found inside high-compression, turbocharged diesels, this material has been used for 30 years or more in many heavy-duty diesel applications (TABLE 5-2). Generally, steel compression rings have the following advantages: ■■ ■■ ■■ ■■ ■■ ■■ ■■

133

better breakage resistance improved heat resistance better mechanical stress resistance reduced ring side wear reduced groove side wear longer service life reduced cost and complexity when being manufactured (made from coiled wire).

With the advent of EGR, ring face materials need to be even harder. Combinations of ceramic metal alloys are now being used. Rings are made harder by nitriding them. Nitriding is a hardening process that impregnates the surface of the steel rings with nitrogen. The hardening extends into the ring to a depth of about 0.001". This hardening technique achieves almost 50% greater wear-resistance than steel rings and four times that of gray cast iron rings. This improves the ring wear resistance to side wear and face wear to the point where ring wear is negligible. Cylinder walls will wear out before these rings do. One other exotic diesel engine ring material uses a combination of plasma moly and ceramics such as chromium carbide. Ceramics are extremely hard and wear resistant, but do not conduct heat well. Moly-Cermet is one such material. It is composed of 80% molybdenum and 20% chromium carbide ceramic, which makes for durable ring face material in a turbocharged diesel engine.

Compression Ring Shapes Keystone-shaped rings are the ring shape of choice for most engine manufacturers (FIGURE 5-42). Rectangular-shaped rings were once the standard ring shape, but they have a problem with sticking when combustion and baked oil and fuel residues carbonize in the ring groove. Keystone-shaped ring action helps keep the ring land clean and helps prevent ring sticking. A barrel face is often ground on the face of compression rings to ensure the ring is properly sealed and to minimize friction. The barrel Compression Ring face forms line contact with the cylinder wall to form tight and relatively Barrel Face friction-free contact. Prior to packaging compression rings, the manufac- Keystone turer laps rings in a cylinder identical to the one that the rings will run in. A witness line left by the lapping process shows the contact with the Intermediate Ring cylinder. This same line will grow wider as the ring wears into its installed Taper Face cylinder. The width of the witness line can be used as a visual indication to determine the amount of ring wear.

Chrome-Molybdenum Ring

Molybdenum Insert FIGURE 5-41 This illustration of a

molybdenum-faced (or “moly”-faced) ring has a porous finish which is used to retain oil aiding upper cylinder wall lubrication. Note that chrome rings are shiny.

▶▶TECHNICIAN TIP The durability requirement for engine life established by emission regulations means that rings must last longer. Rings can be made harder for improved wear resistance but the disadvantage is prolonged break-in periods. Both the ring and cylinder wall must conform to each other’s shape during the initial engine run-in period for optimal cylinder sealing. At one time, break-in would takes place in hours with softer ring materials. Some heavy-duty engines now take as long as 12,000 miles (19,300 km) before they are considered to have been “broken in.” Blowby, low power, and the excessive production of engine slobber are complaints arising from longer break-in periods.

Notch Up

Notch Down

Intermediate Rings

Oil Control Coiled Expander

Intermediate rings might also be called compression rings. However, they do more than compress and seal. While intermediate rings seal compression gases that escape past the top rings, they also assist in oil control. A small, metered quantity of oil is required on the cylinder walls for the piston and rings, but any excess quantity will lead to high oil consumption and emissions. The ring shape of choice for the intermediate ring is a taper-faced

FIGURE 5-42 A keystone-shaped compression ring.

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ring. This shape allows the ring to glide over the oil film on the upstroke, enhancing the sealing effect of oil between the ring and cylinder wall. On the downstroke, the ring will scrape oil from the cylinder back into the crankcase. A variation of this design lets the ring twist in the groove. Using a ring with a groove machined in the bottom or top inside edge of the ring permits gas pressure to cause the ring to twist during either compression or power strokes. Either compression sealing or oil control can be enhanced, depending on the direction of twist (FIGURE 5-43).

Compression Pressure

Compression Ring

Ring Notch

Intermediate Ring

Oil Control Rings

The last ring type found on a ring pack is the oil control ring. This ring is usually cast iron with scraper rails that remove excess oil from the cylinder wall while leaving a metered quantity for lubricating the cylinder wall. This oil film thickness is typically no greater than four-millionth of an inch (0.000004" or 0.0001016 mm) thick. The oil control ring is usually a low-tension-type ring and has no gas pressure acting on it in a significant way. For this reason, a metal expander is placed behind the ring to maintain constant uniform pressure against FIGURE 5-43 Depending on the twist of the intermediate ring, the ring’s oil control or compression sealing function can the cylinder walls by the scraper rails. The expander ring may be corrube enhanced. gated or slotted or a wire coil. Like the intermediate and compression rings, the oil control ring is vulnerable to premature wear from dirt ingestion into the engine. Scraper rails will show premature wear if abnor▶▶TECHNICIAN TIP mal amounts of dirt contaminate the oil through the intake air (FIGURE 5-44).

It is important to note that compression rings and intermediate rings have directional markings to guide the technician during ring installation. Markings are usually located on the top of the ring and orientate the ring to the top of the piston. For the intermediate ring, the direction of orientation is critical since an incorrectly orientated intermediate ring can lead to high oil consumption, excessive blowby, and low compression.

Oil Control

Ring Installation Care must be exercised when installing rings onto a piston and into the cylinder since rings are fragile. A common cause of ring breakage is overstretching the ring during installation. Ring expanders are tools that stretch a ring just far enough for it to be removed but not too far to cause it to bend or break. To prevent loss of compression and damage to the cylinder wall, ring end gaps must be staggered 120° apart in a three-ring pack configuration. End gaps should not be aligned near the wrist pins (FIGURE 5-46). If rings are reused such as after performing warranty repairs on engines having accumulated low travel distances, extra caution should be exercised when removing pistons from cylinders to prevent ring damage. The wear step or cylinder ridge found above the ring travel area in the cylinder should be first removed to prevent damage to the rings as well as the piston when both the piston and rings are pushed up over the ridge during removal. Failure to remove the cylinder ridge by cutting or grinding it away will score the piston and even break the rings. Use ring compressor to install a piston into the cylinder and prevent damage to the rings (FIGURE 5-45). A number of designs are available to ease ring installation.

▶▶ Cylinder

Component Service

While diesel engines provide two to four times longer service than comparable spark-ignition engines, the engines are often installed in a chassis or equipment with even longer life. Off-road equipment and on-highway vehicles operating long hours or distances every day will accumulate high engine mileage before the rest of the equipment FIGURE 5-44 Oil control rings consist of scraper rails and an will wear out. Engine repair or overhaul on high-value machines is expander ring to maintain rail contact with the cylinder wall. justified in these circumstances. Occasionally, engine failures may need to be replaced at a component level too. More often than not, labor costs, warranty policy, the cost of downtime, and available skill level for repairing the engine a guiding 5-05 Identify and describe diagnostic factors for the decision as to whether to repair or replace. However, technicians should be techniques used for evaluating the familiar with some of the common service and inspection procedures for cylinder service if condition of a piston ring–to–cylinder they find themselves called upon to repair rather than replace an engine. wall seal.

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Cylinder Component Service

135

Ring Bands Tightening Handle

Ratchet

Band Pliers

FIGURE 5-45 Various types of piston ring compressors are used to squeeze rings on the piston while inserting the

piston into the cylinder.

Evaluating Cylinder Sealing The effectiveness of seal between the rings and cylinder wall is an important measure of engine life. A poor seal means the engine is worn out or has internal damaged. The question a technician needs to answer to recommend whether a repair should be made is, how well are the rings sealing against the cylinder wall seal? This means that the ability to quantify or measure sealing effectiveness is an important skill. Three methods are available to the technician to evaluate the condition of piston ring–to–cylinder wall seal. 1. Compression testing. Compression testing is rarely used on diesel engines, due to the lack of clearance volume that the cylinders have and due the prospect of the cylinders being hydraulically locked with oil. Wide variations of compression results are possible due to the design of piston rings and low clearance volume in a diesel. Furthermore, it’s cumbersome to remove an injector to install a compression gauge into a cylinder. However, glow-plugs in smaller, lightduty diesel engines can be removed to test compression pressure. Other methods to evaluate cylinder sealing are more effective though. Maximum compression pressure is measured with a pressure gauge installed in the injector or glow-plug hole. When the engine is cranked over, maximum pressure values are observed for each cylinder and compared against manufacturer’s specifications as well as for cylinder-to-cylinder variations. Compression pressures should not vary by more than 10% between cylinders. Compression testing is more commonly used in spark-ignition engines (FIGURE 5-47). 2. Cylinder leak-down testing. A more accurate and meaningful measurement of the cylinders sealing capabilities can be obtained from a cylinder leak-down test. This test can be performed separately or in conjunction with compression testing and is more practical for diesel engines. Unlike compression testing, air is fed into a cylinder at pressures between 80 and 120 psi (between 551.6 and 827.4 kPa). The engine should be warm to minimize clearances between the cylinder walls and the piston. With the valves closed, the piston is positioned at top dead center (TDC) and the engine locked in this position. The cylinder leakage tester is connected to the cylinder and

Top Ring Gap

2nd Ring Gap

Bottom Ring Gap FIGURE 5-46 Piston ring end gaps must be staggered 120°

apart to prevent gases from easily leaking past the rings.

FIGURE 5-47 Diesel compression testing gauge with a variety

of glow-plug and injector adapters.

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Chapter 5 Cylinder Components

Regulated Pressure Gauge

Cylinder Leakage Gauge

Flex Hose

Air Supply Pressure Regulator

0.040" Dia Orifice Whistle

Quick Connectors Injector Adaptor

FIGURE 5-48 A leak-down tester supplies regulated air pressure to a cylinder and measures the percentage of regulated

air pressure maintained in the cylinder. The percentage of leak-down is an indicator of cylinder and ring condition.

pressurized with regulated shop air. A second gauge on the leakage tester will record the percentage of air leaking into the cylinder (FIGURE 5-48). The rate of leakage is compared against manufacturers’ specifications to determine whether the amount of leakage is acceptable. Engine problems can be detected by listening for large volumes of air leaking out of the turbocharger, intake or exhaust system, or even the oil filler tube. An engine in good condition should generally show only 5–10% leakage in the pressure of air supplied to the cylinder after a minute. Some engines in acceptable condition may indicate as high as 20% leakage. But more than 30% leakage indicates problems. A cylinder that has poor compression but minimal ring leakage may indicate a valve-train problem such as a worn cam lobe, broken valve spring, inadequate valve lift due to a defective cam follower, or even a bent push rod. If all the cylinders have low compression but show minimal leakage, the most likely cause is incorrect valve timing. A slipped notch on a timing belt or timing chain or an incorrectly installed cam gear may be the problem in this instance. A damaged valve will leak air from either the intake or the exhaust pipe. If compression is good and leakage is minimal but a cylinder is misfiring or shows up weak in a power balance test, it may indicate that the diesel engine has a defective injector or injection pump. 3. Crankcase blowby. One of the best recommended practices for evaluating the condition of piston rings in a diesel engine is to test crankcase pressure. Since blowby gas pressure in the crankcase is a function of the effectiveness of the ring-to–cylinder wall seal, the quantity of blowby and crankcase pressure is a good measure of ring-to–cylinder wall condition. Crankcase blowby pressure is usually very low—just a few inches (centimeters) of water column pressure when the engine is under full load (rings are seated). When the rings and cylinder walls are worn, blowby volume increases, which drives up crankcase pressure. Blowby testing is a fast, simple, and unobtrusive technique to evaluate the condition of piston ring–to–cylinder wall sealing. Pressure is measured at high idle, no-load operation. When piston rings seal well, only a little blowby gas enters into the crankcase. When cylinder pressures are higher, such as during high-idle-rpm operation, the rings seal much better

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Cylinder Component Service

137

SKILL DRILL 5-2 Performing a Crankcase Blowby Test

1. Before starting the crankcase pressure test, it is important to run the engine until it reaches normal operating temperature. A cold engine will produce an abnormally high pressure reading. 2. Install the crankcase orifice restrictor tool in a road draft tube or a crankcase pressure test adapter in the oil fill hole. Use the manufacturer’s prescribed tool and restriction diameters.

3. Ensure that the oil dipstick tube seal and any other gaskets and seals on the engine are functioning properly. 4. Connect the Magnehelic gauge that has plastic tubing to the restriction tool. There are two ports on the gauge: pressure and vacuum. Connect the restriction tool attached to the crankcase outlet to the pressure port of the Magnehelic gauge. 5. Make sure the gauge tubing is clean and dry so that the gauge does not become damaged. It is good practice to blow clean, dry compressed air through the tubing before each use. 6. Zero the Magnehelic gauge. 7. Chock the wheels of the vehicle and apply the brakes. Place the transmission in neutral or park. 8. Start the vehicle’s engine and run it at wide-open throttle under no load. Maintain this engine speed for at least 30 seconds and take a stabilized reading. Make sure the hole in the top of the crankcase orifice restrictor tool is never blocked when the engine is running. 9. Record the results and compare them against specifications. Any readings of more than 4" (102 mm) of water during the crankcase pressure test indicates a worn engine. It is likely that base engine mechanical concerns exist. Make a service recommendation to overhaul the engine if engine hours and distance traveled are high.

than they would at low-engine rpm. In a good engine, crankcase pressure is more often higher at idle than at high idle. However, glazed cylinders are indicated when crankcase pressure is significantly higher at idle and drops as engine speed increases. To perform a crankcase blowby test, follow the steps in SKILL DRILL 5-2. Pressure in the crankcase is measured in inches of water column or kilopascals by using a water manometer or Magnahelic gauge (FIGURE 5-49). Measurement units for crankcase pressure are very sensitive measurements of pressure. The units of measurement and the tool used to make the measurements are also often not familiar to many technicians. To develop a sense of how sensitive they are, and to avoid the mistake of using other pressure gauges or units of measurement, see TABLE 5-3. A calibrated restriction fitting is placed over the crankcase breather tube. If equipped, the crankcase depression regulator will be blocked during the test. After the engine coolant and oil are warmed to operating temperature, the engine is put to full throttle, no load and crankcase pressure is measured. Usually only a few inches of pressure are measured in an engine in good condition. Small-bore diesels will have a maximum limit of 10" of water column pressure (2.49 kPa). Larger engines may be allowed as much as 15–18" of pressure (3.73–4.48 kPa) (FIGURE 5-50).

Measuring Cylinder Wear In addition to unobtrusive measurements to evaluate the effectiveness of the ring-to–cylinder wall seal, direct measurement of the cylinder can indicate the amount of engine wear to provide a service recommendations.

FIGURE 5-49 A Magnahelic gauge measures pressure in inches

of water column (or kilopascals). These gauges are much more sensitive to pressure changes than measuring using with mercury.

TABLE 5-3 Pressure Equivalents 1 bar =

1.02 atmospheres

29.53" of mercury

401" of water

14.5 psi

1,000 kPa

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Chapter 5 Cylinder Components

Tapered and Out-of-Round Cylinders In slightly tapered and out-of-round cylinders, oil consumption is controlled by the pistons and rings. However, with increased cylinder taper and out-of-round conditions, good oil control in the cylinders is difficult to maintain due to a combination of several factors. First, increased piston clearances allow pistons to rock in the worn cylinders. This condition allows an abnormally large volume of oil to enter on one side of the piston. If the rings are also tilted as the piston rocks, oil can enter the combustion chamber and burn when the piston reverses direction and moves downward (FIGURE 5-51). Second, at high piston speeds, the rings may not have adequate time to conform perfectly to all the worn parts in the cylinders on every stroke. Whenever this occurs, the engine burns larger quantities of oil that have remained on the cylinder walls.

FIGURE 5-50 Measuring crankcase blowby

pressure involves the use of a restrictor orifice and a Magnahelic gauge measuring inches of water column pressure or kilopascals. Blowby is measured on a warm engine operated at highidle rpm.

Distorted Cylinders

Cylinders that are distorted so that they are out of shape from other causes, such as unequal heat distribution or unevenly tightening the cylinder head bolts, may distort the cylinders. Cylinder distortion limits the piston ring’s ability to scrape the cylinder walls completely. Another type of distortion is cylinder taper, which is caused by cylinder walls undergoing uneven wear. Taper, which is widest near the top of the cylinder where the compression ring turns around, becomes progressively smaller near the bottom of the cylinder. Taper wear is caused by several factors. The most significant is the absence of upper cylinder wall lubrication, where temperatures and compression ring pressure are highest (FIGURE 5-52). In the ring turnaround area, above the oil control ring’s uppermost limit of travel, only lubricant on the compression and intermediate ring face reduces cylinder wall friction. Gas pressure behind the rings creates the highest unit loading of the ring face. Below the ring turnaround area, the presence of more lubricant on the cylinder wall reduces friction and wear. Cylinder taper and out-of-round conditions can be evaluated by measuring cylinder dimensions with a dial bore gauge or ring end gap method. By installing a ring into a cylinder in three different places—one in the bottom, middle, and ring turnaround area— and then measuring the end gap, an approximate measurement of cylinder wear can be determined. The ring should be squared or perpendicular in the cylinder by using a piston

Oil scraped into combustion chamber caused by piston tilt.

FIGURE 5-52 The area above the intermediate ring but FIGURE 5-51 Wear points on a worn piston. Worn pistons tilt

and scrape oil up into the combustion chamber.

below the compression ring experiences the greatest wear due to lack of upper cylinder wall lubrication. This is the ring turnaround area.

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Piston Pins

139

SKILL DRILL 5-3 Measuring Piston Ring End Gap and Ring Groove Wear

1. With the piston removed, clean the ring groove by using a soft nylon bristle brush and a non-caustic cleaner. Alternatively, using a broken piece of ring, clean carbon from the ring groove by working a section of broken ring through the groove. Be sure to wear protective gloves. 2. Using a new ring, place the outside circumference of the ring in the ring groove. 3. Using flat feeler blades, measure the side clearance of the ring by sliding the largest possible feeler blade between the top of the ring and the groove. 4. Compare specifications against the service manual. Generally, small-bore engines allow no more than 0.002" or 0.003"

(0.0508 mm or 0.0762 mm) and larger pistons will be allowed as much as 0.01" (0.254 mm). 5. To measure ring end gap, place the ring inside the cylinder. 6. Square the ring in the bore with the top of a piston so that the ring is perpendicular with the cylinder wall. 7. Select the largest thickness of feeler blade that will fit between the ends of the piston ring. 8. Compare the end gap measurement against specifications. Generally, 0.004" (0.1016 mm) of end gap is required for every inch (2.54 cm) of cylinder bore diameter.

before measuring the end gap with a feeler blade. Since the ratio of cylinder diameter to radius is 3.14, the differences between the maximum and minimum end gap should be divided by a factor of 3.14. In other words, a difference of maximum and minimum end gap of 0.008" ÷ 3.14 = 0.0025" taper. Piston rings can wear and their condition can be evaluated by examining the ring face. A barrel-faced ring will have a wide line worn into it in comparison to the narrow contact line on a new ring. Taper-faced rings will similarly have a wider, “shinier” line in comparison to a new ring. Rings ends will be thinner and will taper rather than have a square appearance. However, ring grooves can wear due to the continuous flexing of rings from a combination of reciprocating action and gas pressure. Several types of gauges are available to use to inspect the groove shape and wear, but one alternative method is to measure the clearance between a new ring and the ring groove. To measure piston ring end gap and inspect for excessive compression ring groove wear, follow the steps in SKILL DRILL 5-3.

▶▶ Piston

Pins

Piston pins are the connecting joint between the connecting rod and the piston. Piston pins may be either hollow or solid and are fabricated from high-alloy steels. The area of the piston pin bore of a piston is referred to as the pin boss. This area is reinforced since the thrust forces of combustion are transmitted through the piston to the pin through that area.

5-06 Identify and explain the function of piston pins.

Semi-floating Pins There are a variety of piston pins designs. One design is the semi-floating wrist pin. This design requires the piston pin to be pressed into the connecting rod and piston bore. In this arrangement, only the piston rotates on the pin while the pin is fixed by an interference fit with the rod. Assembling and disassembling the pin requires several tons of force and using a special fixture to prevent damage to the piston. The advantage of this pin design is that it

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Chapter 5 Cylinder Components

does not have the problem of clips loosening and falling out. This configuration is not commonly used on diesel engines, since wear is localized to the pin and connecting rod bushing.

Full Floating Piston Pins

FIGURE 5-53 Hollow and solid wrist pins. Lower

cylinder pressures mean that hollow pins can be used. Offset Pin Centerline

Piston Centerline

Thrust Side

FIGURE 5-54 The piston pin axis offsets from the

cylinder centerline to eliminate noises caused the piston rocking at TDC. Doing this allows the piston to change sides before it reaches TDC, minimizing noise.

The piston pin design common to most diesel engines is a full floating piston pin. Both the piston and the connecting rod rotate on the pin. Although this requires a bushing in the small end of the rod, more surfaces are rotating with less load with an oil film to better distribute wear. This type of pin is held in place by spring clips or, in the instances of some newer engines, Teflon caps that prevent the pin from coming out of the bore and scratching the cylinder. Some wrist pins may have an interference fit when at room temperature but the pin bore will increase in diameter with temperature in a running engine to allow the pin to move freely on both piston and con-rod. This type of pin needs to be heated while removing and replacing it (FIGURE 5-53).

Wrist Pin Offset Although the piston moves linearly in the cylinder, the angle formed between the connecting rod and the crank throw pushes the piston sideways as it converts the reciprocating piston motion to crankshaft rotational motion. This push is known as side thrust and is divided into a major and minor thrust. The amount of thrust is a function of the angle formed between the connecting rod and the piston and cylinder pressure. Major thrust is produced during power stroke and minor thrust during compression stroke. Long stroke engines, like diesels, have greater thrust because of the angles formed between the long crank throw and longer connecting rods. The problem of the side thrust combined with higher cylinder pressures can cause three problems in diesels engines: 1. Higher side thrust forces can lead to excessive cylinder wall wear. 2. Cylinder wall and liner cavitations may appear on the coolant side of the cylinder wall. This is a problem unique to diesel engines, where pitting on the coolant side of the cylinder wall can penetrate the cylinder. This phenomenon is explained in more detail in Chapter 7, on cooling systems. 3. Noise can be heard as the piston crosses over from pressing against one side of the cylinder to the other near TDC. Clearances between the sides of the piston instantly switch from one side to the other side as the piston moves through TDC (FIGURE 5-54).

To minimize the degree of side thrust and minimize noise caused by piston slap, engine manufacturers often offset the centerline of the wrist pin with the centerline of the piston. Because the centerlines of both the piston and wrist pin are not the same, the piston must have a specific orientation in the engine. This means there is a side of the piston that must be oriented to the left or right side of the engine. The purpose of wrist pin offset explains why pistons have markings to ensure that they are installed in the correct direction in the engine. Markings are usually present on the piston to indicate its orientation toward the front, rear, or side of the engine (FIGURE 5-55). Another strategy to reduce the thrust angles in the engine is to offset the centerline of the crankshaft with the engine centerline. By moving the centerline of the crankshaft and wrist pin slightly to one side, the thrust angle is reduced. What this means for the technician is that connecting rods and pistons have a specific front/back left right orientation in an engine. For example, pistons must be orientated typically with a reference to the camshaft or front of the engine. Similarly, connecting rods often have directional indicators or stamping marks facing the front of the engine.

FIGURE 5-55 Note the directional arrow on this piston crown. Thrust forces are minimized by

offsetting the wristpin’s centerline. For this reason, pistons have a specific orientation in the engine.

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Connecting Rods

▶▶ Connecting

141

Rods

Connecting rods link the piston to the crankshaft enabling the conversion of the pistons reciprocating motion into rotational movement of the crankshaft. Connecting rods are the most stressed component of the engine since they experience centrifugal tensioning and compression loading every time the piston changes direction (FIGURE 5-56).

5-07 Identify and explain the function of connecting rod construction features and mechanical stresses imposed on connecting rods.

Compression and Tensional Loading Tensional loading occurs at the end of the end of upstrokes at TDC, where the weight of the piston assembly would want to continue in a straight line through the cylinder head. Without the cushioning effect of pushing exhaust gases out or compressing the gases in the cylinder, the tensional forces would be even greater. Compression loading occurs during power stroke, where the combustion forces push the connecting rod down with forces, in excess of 15 tons of force (206.8 mPa). For this reason, connecting rods are built to typically withstand forces greater than 16 times what is normally encountered. These forces could be from abnormal combustion conditions or engine over-speeding. Engines using heavier steel or articulated pistons have larger-sized con-rods to handle the increased tensional forces (FIGURE 5-57).

Connecting Rod Materials Since the con-rods are highly stressed, they are commonly forged from high-alloy steel. Rods are then shot peened to relieve stress risers and add additional fatigue strength. Bolts used on the caps are made from proprietary alloys of grade-eight metals. Rolled threads are used on these bolts to increase their strength (FIGURE 5-58).

Rod Cap

Bearing Locating Tabs I-Beam Cross Section

Balance Pad

Piston Bin Bushing

Balance Pad

Locating Dowels

Big End

Forging Parting Lines

Small End

FIGURE 5-56 Connecting rod features and nomenclature.

FIGURE 5-57 Connecting rods use bolts with stronger rolled

threads and propriety grades of hardness. Rolled threads have a larger root diameter than the equivalent conventional threaded bolts.

Rolled Thread

Conventional Thread

FIGURE 5-58 The root diameter of a rolled thread is larger than

a conventional thread.

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Chapter 5 Cylinder Components

Connecting Rod Features

FIGURE 5-59 The teepee style of connecting rod is

used most commonly in diesel engines since it possesses better wear and loading characteristics.

Connecting rods will use an I-beam style cross section, which has good weight-to-strength characteristics and resists bending. The smaller end of the rod will have a bushing pressed into it if it is used with a full floating piston. When the small end is shaped like a triangle, it is called a teepee-style rod (FIGURE 5-59). The weight of a set of connecting rods is critical to engine balance. If connecting rods vary in their mass, an engine vibration will result. To adjust the weight of connecting rods, balance pads are located at each end for grinding, to balance rod weight between rods and around its center point. Select fit rods are often used together in sets in an engine. This means that rods are weighed after they are manufactured and that those closest in weight are used together in sets. These can be identified by the paint marks that uniquely identify the weight variation of the rod. For example, a group of connecting rods may have white, yellow, red, or other paint markings on them that can designating whether they belong to a particular weight classification. Such rods sets should be kept together when assembling an engine. If a service rod is required, it will often have a paint marking that is in the middle of the weight distributions.

Fracture-Split Connecting Rods

Fracture-split connecting rods are the most commonly manufactured type of connecting rods today. These rods have the big end of the rod parting surfaces that are split by fracturing rather than machining the mating surfaces between the rod and bearing cap. Using a fracture-split rod cap offers several advantages: ■■ ■■ ■■ ■■

more precise alignment of the rod and cap than a machined rod and cap improved control of oil clearances reduced production cost since machining steps are eliminated greater retention strength between the cap and rod.

Fracture-split rods allow the rod and cap to be separated along a predetermined fracture line. Instead of cutting the rod cap away from the rod, then machining and re-boring the rod and cap, a hydraulically operated wedge pushes the two pieces away from one another. A line scribed where the split should take place provides a weakened point to control the split. Since the big end of the connecting rod is fracture-split, the rough-edged of the big end and cap have greater retention strength than a smoother conventional flat-machined surface (FIGURE 5-60). Fracture-split rods can be made to be about 17% lighter, reducing engine vibration while maintaining the strength of traditionally manufactured rods. Fracture-split rods are also produced more economically and by using less energy during production.

FIGURE 5-60 Fracture-split rods have a coarse grainy appearance on the bearing cap parting halves.

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Connecting Rods

Connecting Rods and Other Unique Features One common feature is that rod ends can have an offset or split of the rod cap at 45°–60° rather than at a conventional 90° (FIGURE 5-62). This feature is to accommodate larger rod journal sizes while allowing the rod to pass through the cylinder during assembly. If the rod were not split at an offset angle, the big-end size would not pass through the cylinder, because of the direction of the orientation of the rod bolts. The mass of the large end can be somewhat minimized by using this feature. Diesel engines will often have a drilled passageway through the rod to the wrist pin bushing to supply oil to the wrist pin. In V6 and V8 engines, connecting rods share a common crankpin journal, which means rods must be correctly orientated. A chamfered edge in the rod cap is installed toward the crankshaft fillet radius—the edge between the journal and crank cheek. The flat sides of each rod match one another, allowing a surface for each rod to glide over one another.

Connecting Rod Servicing

143

▶▶TECHNICIAN TIP If the con-rod cap of a fracture-split rod is fitted incorrectly and tightened, then the connecting rod must be replaced. This is necessary because the unique profile of the mating surfaces will have been damaged when the cap tightened causing distortion of the two mating surfaces. The cap will no longer locate correctly, even if it is returned to its correct position. To minimize the possibility of this error, connecting rod bolts or cap screws are located off-center, to ensure that the cap is fitted to the rod in the correct orientation (FIGURE 5-61).

Connecting rods should be checked for twists, bends, wear, nicks, and cracks, as well as how clear the oil passageways drilled through the rod are (FIGURE 5-64). Twists and bends are usually due to hydrostatic locking. A hydraulic lock occurs when liquids entering the combustion chamber eliminate the little clearance volume remaining in a diesel engine. When the piston compresses the liquid, it will stop moving, and the crankshaft continues to move, causing the con-rod to bend and twist (FIGURE 5-65). When this happens, the engine is said to be locked up. Sources of liquids entering the combustion chamber include head gasket leakage; block porosity (i.e., cavitation erosion); leaking injectors or injector tubes; or water entering the

FIGURE 5-61 Locating dowels rod

cap and rod must match one another and line up together to properly assemble a cap and rod.

▶▶TECHNICIAN TIP FIGURE 5-62 An offset split rod permits a larger crankpin

diameter and a stronger rod cap retention. Loads are carried by the bolts and fillets machined into the rod.

FIGURE 5-63 When assembling any rod and cap, markings must

Connecting rods caps are line bored (drilled with a large drill bit) when manufactured and cannot be reversed during installation; otherwise, the bore will not be concentric (FIGURE 5-63). Technicians must carefully observe match marks used to put the cap and rod together properly during assembly. Other markings are used to indicate a front-to-back orientation in an engine since an offset angle may be present in the con-rod. On V-block engines, two connecting rods share a single crankshaft journal. One side of the con-rod is chamfered to prevent damage to the crankshaft fillet radius. The other side of the same rod will be smoothly machined to allow the connecting rods to rotate against one another.

match one another and align together.

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Chapter 5 Cylinder Components

Little End

Oil Passageway

Big End

Serrations

Bearing Shell FIGURE 5-64 A piston pin lubrication hole drilled

FIGURE 5-65 This connecting rod was bent due to a hydrostatic lock

through some rods needs to be inspected and cleaned.

in the cylinder.

▶▶TECHNICIAN TIP Whenever any major engine work is performed, always turn the engine over manually first before using the starter. This can identify incorrectly installed, misadjusted components or the presence of hydraulic locks, which could potentially damage the engine if the starter is used.

exhaust system through an open exhaust stack or water, snow, oil, or fuel entering through the intake system. For this reason, when servicing an engine with a procedure that could cause a hydrostatic lock, such as during an injector replacement, it is important to bar the engine over manually and determine whether the engine is locked up.

Inspecting Connecting Rods To evaluate whether a connecting rod is bent or not, a technician may first locate a misfiring cylinder due to low compression. Sometimes a rod may be bent far enough that the skirt hits the crankshaft counterweight, creating a tremendous bottom-end noise. If the cylinder head is removed, a simple check of piston protrusion above or below the block deck may be enough. All pistons should have relatively the same amount of protrusion if the rods are the same length (FIGURE 5-66). Finally, a rod can be removed and measured in a machine that will evaluate the length and degree of parallelism between the big and small end.

Crack Detection Connecting rods may be checked for cracks by using dye penetrant or magnetic particle detection. Stresses encountered during operation can lead to stress risers opening and a catastrophic engine failure. Machine shops can also check connecting rod bores for roundness and wear. Worn bushings in the small end can be replaced.

FIGURE 5-66 Measuring piston

protrusion above the block deck is one method to check for a bent connecting rod. A bent rod will have less protrusion than other cylinders.

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Wrap-Up

145

▶▶Wrap-Up Ready for Review ▶▶

▶▶

Cylinder components consist of the pistons, piston rings, piston pins, and connecting rods. Because these components convert combustion energy into mechanical power, they are the most highly stressed parts of the diesel engine. Cylinder components need to have a useful life that meets or exceeds EURO 6 and EPA emission standards for durability. The EPA requires 150,000 miles (241,400 km) or 15 years for Tier 3 engines to remain emission compliant. Basic piston designs used in diesel engines include slipper skirt, trunk, articulating (medium-duty), and Monosteel (light-, medium-, and heavy-duty) pistons. Slipper skirt pistons are used in lighter, more compact diesels, whereas Monosteel pistons are used in the a few light-duty but mostly medium- and heavy-duty engines. Articulating pistons have a two-piece design and use an aluminum skirt attached to a steel crown with a piston pin. Trunk pistons are longer than they are wide. Low crevice volume (LCV) pistons reduce emissions by minimizing the space between the piston crown and cylinder wall above the compression ring. Articulating and Monosteel pistons are classified as LCV pistons because the top ring is closer to the crown and because the clearance between the cylinder wall and crown is reduced. Although aluminum pistons are traditionally lighter than steel pistons, which minimizes inertia forces at the end of each stroke, aluminum pistons have several disadvantages. Aluminum pistons need high crevice volume and larger operating clearances, and this has given rise to pistons with steel alloy crowns, which can withstand higher cylinder temperatures and pressures, to displace aluminum pistons in many applications. Pistons are cooled by several mechanisms, including by transferring heat through the piston rings during valve overlap, when the intake air mass crosses over the piston, and by oil cooler nozzles, which spray engine oil aimed at the underside of the piston. Piston rings help form a gas-tight seal in a cylinder, distribute oil over the cylinder wall, transfer heat from the piston, and provide a low-friction, replaceable wear surface. Three types of piston rings are used in an engine: a compression ring, an intermediate ring, and an oil control ring. Each ring has its own unique shape and materials based on its primary function. Compression rings help form a gas-tight seal in the cylinder. Intermediate rings form a compression seal and control oil distribution. Oil control rings distribute a thin coat of oil onto the cylinder walls. The introduction of low-tension, low-friction piston rings is the most recent change in ring technology to reduce engine wear and friction.

▶▶ ▶▶ ▶▶

▶▶ ▶▶

▶▶

Steel piston rings are more commonly used. Longer breakin periods are required due to harder ring face materials. Piston pins are offset from the piston’s centerline to reduce noise when the piston crosses over TDC. Piston rings, pistons, and connecting rods all have directional markings that indicate which way they should be installed in an engine. Rolled threads, which are stronger than machine-cut threads, are used on connecting rod cap bolts. Prolonged engine idle produces engine slobber, which is a combination of lubricating oil, unburned fuel, and soot. Slobber can cause piston rings to stick and leave damaging deposits on pistons. Prolonged engine idle time can also increase cylinder glazing and soot loading in engine oil. Glazing takes place when the fine lines on the cylinder wall, called crosshatch, are filled with oil that is then baked and smoothed over by the piston rings. When glazed, cylinder walls cannot properly retain oil, which leads to increased cylinder blowby and power loss. Crankcase pressure testing is the most effective way to unobtrusively evaluate how much an engine’s cylinders are worn. Most wear takes place in the cylinders in the ring turnaround area, which is the place in the cylinder above the highest point of travel in the oil control rings and below the compression ring. Cylinder wear is tapered, with the greatest wear at the top of the cylinders. After rebuilding an engine, the engine must be run in under a heavy load to seat piston rings and minimize the likelihood of cylinder glazing.

Key Terms anodizing A process used to harden aluminum by electrochemically reacting oxygen with aluminum. articulating piston A two-piece piston design that uses a separate aluminum skirt connected to an alloy steel crown through a piston pin. break-in period The operation of an engine after it is initially assembled or rebuilt when piston ring, cylinder wall, bushing, and bearing surfaces have high initial wear as the moving surfaces conform to each other. cam ground piston An elliptically shaped piston that expands to a round, symmetrical shape after it is warmed up. compatibility A property that allows two metals to slide against one another with minimal friction or wear. compression testing A measure of the maximum pressure of engine cylinders when cranking. crankcase pressure test A measurement of the amount of cylinder blowby. This indicates the sealing ability of the piston rings and cylinder walls.

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Chapter 5 Cylinder Components

crevice volume The area between the piston crown and cylinder wall above the top compression ring. crosshatch A cylinder wall finish of fine intersecting lines that are used to retain oil. cylinder glazing A condition that occurs when lubricating oil is first baked and then smoothed into the crosshatch of the cylinder wall. Cylinder glazing prevents the cylinder wall from retaining oil and reduces ring’s ability to seal. cylinder leakage The percentage of gas leakage past the rings. High leakage rates are usually due to worn cylinders and rings. dial bore gauge A precision measuring tool used to measure taper wear in a cylinder. durability requirements Legislated standards for engine durability that require noxious engine emissions to remain below set thresholds through the expected useful life of an engine. dusting out A condition where dirt is drawn into an engine. This causes premature abrasive wear on the cylinder walls and rings. forged pistons Pistons made from aluminum alloy billets that are stamped into shape by forging dies. hydrostatic lock A condition that occurs when fluids in the engine’s cylinder prevent the engine from rotating. hypereutectic piston A piston that has high silicon content (16–20% silicon). Magnehelic gauge A gauge that measures both pressure and vacuum by using inches or a water column or kilopascals as a measuring unit. major side thrust A piston side thrust caused by cylinder pressure and the angle of the connecting rod during power. micro-finish etching A piston skirt finish where many fine lines are machined into the skirt to retain oil. minor side thrust Piston side thrust caused by compression pressure and the angle of the connecting rod. Monotherm piston A one-piece piston design made entirely of alloyed steel, which has a compact height and a large reduction of material between the skirt and crown. Ni-Resist insert A stainless steel–nickel alloy insert placed in aluminum pistons in the compression ring groove. The insert minimizes groove wear caused by ring movement. nitriding The process of hardening a metal’s surface by heating the metal and quenching it with cyanide salts. piston slap A noise in the engine produced by large operating clearances between a piston and cylinder wall. Piston slap is most often heard when an engine is cold. shot peened A technique that uses small steel balls to blast metal surfaces in order to close up any small cracks or pores, which have the potential to become larger. slipper skirt piston A piston design that has a portion of the skirt removed on both non-thrust sides of the piston to provide clearance for the crankshaft counterweights.

Review Questions 1. A piston’s compression ring seals best during a(n) __________. a. compression stroke b. power stroke c. intake stroke d. exhaust stroke 2. Which ring will cause excessive oil consumption if it is installed upside-down? a. The compression ring. b. The intermediate ring. c. The oil control scraper ring. d. Both the intermediate and compression rings. 3. If an engine has a noise when it is cold but the noise disappears when the engine is warmed up, what is a likely cause of the noise? a. A piston installed on the connecting rod is in the wrong direction. b. An incorrectly installed piston skirt on an articulating piston. c. Piston slap. d. A loose connecting rod bushing. 4. Which of the following is the practice recommended to evaluate how well a diesel engine’s piston rings seal? a. A cylinder leak-down test. b. A compression test. c. A wet-dry compression test. d. A measurement of the crankcase pressure. 5. Which shape is most often used for compression rings in medium-duty diesel engines? a. Square. b. Rectangular. c. Bevel faced. d. Keystone. 6. Where is a nickel insert used in an aluminum alloy piston used by diesel engines? a. Pin boss. b. Intermediate ring groove. c. Skirt. d. Compression ring groove. 7. Which ring face material is used on compression rings to aid upper cylinder wall lubrication? a. Steel. b. Chrome. c. Molybdenum. d. Cast iron. 8. If an engine has 30% loss of crosshatch near the top of each cylinder, approximately what percentage of remaining service life does the engine have? a. 30%. b. 50%. c. 20%. d. 0%.

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Wrap-Up

9. What kind of useful information is supplied by measuring the distance a piston protrudes above the cylinder block deck? a. The amount of liner protrusion. b. The length of the stroke. c. Whether the connecting rod is bent. d. The required thickness required for a head gasket. 10. Under which of the following conditions is the most engine slobber produced? a. Idle conditions. b. Light-load engine operation. c. Full-load engine operation. d. High-speed, light-load engine operation.

ASE Technician A/Technician B–Style Questions 1. Technician A says that when a diesel engine idles excessively, it will cause its cylinder walls to glaze and piston rings to stick. Technician B says idling an engine regularly for long periods of time causes piston rings to break. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that oil control rings are designed to scrape as much oil as possible from the cylinder walls. Technician B says oil control rings leave a metered amount of oil on the cylinder walls. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says compression rings in a diesel engine are rectangular to extend engine life. Technician B says that the top compression rings have a barrel face to provide effective sealing. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that piston ring markings identify the type of ring and its location in a piston ring groove. Technician B says ring markings are used to ensure the rings are installed facing downward. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that after rebuilding an engine, the engine should be operated at low load and low speed to prevent any catastrophic damage. Technician B says that an

147

engine should be operated under a heavy load as soon as possible after ring replacement. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. After discovering broken rings in an engine, Technician A says that the rings were most likely broken due to improper installation. After discovering the same broken rings in an engine, Technician B says the rings were broken due to excessive cylinder taper. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that most high-horsepower (high- kilowatt) diesel engines use trunk-type pistons. Technician B says that most high-horsepower (high-kilowatt) diesel engines today use steel pistons. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 8. Technician A says a scuffed piston skirt and burned piston crown in one cylinder of a recently overhauled engine is likely due to a damaged or improperly aligned oil cooler nozzle. Technician B says the engine was overheated. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. While two technicians are discussing the cause of a cracked piston crown with a vertical crack through the inner edge of the combustion bowl, Technician A suggests that the piston has a manufacturing defect. Technician B suggests that using too much starting fluid cracked the piston. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that a compression test of an engine’s cylinders is the best method to use to diagnose a worn-out engine. Technician B says that measuring crankcase pressure is the best method to identify a worn-out engine. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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CHAPTER 7

Diesel Engine Cooling and Lubrication Systems Learning Objectives After reading this chapter, you will be able to: ■■

■■

7-01 Identify and describe the functions of the diesel engine cooling system. 7-02 Identify and describe the construction and operation of cooling system components. 7-03 Identify and describe the functions of a lubrication system.

■■

■■ ■■

7-04 Identify and explain the classification systems for engine oil according to oil properties and characteristics. 7-05 Identify and explain factors that limit oil service intervals. 7-06 Identify and explain the construction and operation of pressurized oil circuits.

You Are the Technician A number of delivery trucks you service at your repair center ship time-sensitive loads, which means there is almost no tolerance for breakdowns on the road. To ensure these vehicles do not experience any kind of mechanical problems when they’re operating, you have been tasked with developing a preventive maintenance schedule and identifying inspection and service criteria to present to customers as part of a maintenance agreement. Part of preventive maintenance involves cooling and lubrication system service. As you prepare the maintenance schedule, consider the following:

1. Identify potential problems with the cooling and lubrication system that could lead to a breakdown or road service call. 2. Outline maintenance practices and inspection points of the cooling and lubrication system that will require attention during preventive maintenance services. Explain your service recommendations.

3. Outline some of the more important factors of cooling and lubrication system maintenance that promote extending engine life and reliability.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

▶▶ Introduction Diesel combustion characteristics, the type of fuel burned, turbocharging, and high cylinder pressures create additional tasks for the cooling and lubrication systems. These engine support systems have unique construction features and operating requirements to enable the engine to perform all it’s functions well and deliver long and reliable engine operation.

▶▶ Functions 7-01 Identify and describe the functions of the diesel engine cooling system.

of a Cooling System

The following points outline the functions of a cooling system. 1. It removes excess heat. The flame temperature of diesel fuel is approximately 3,900°F (2,150°C). Continuous engine operation at temperatures like this without some means to remove heat would quickly result in damaged and destroyed engine components. Operating clearances between moving engine parts would disappear and cause engine seizure as components thermally expand. Components such as pistons and valves would soften and permanently distort under the high temperatures. Lubrication oil would lose viscosity and fail to maintain separation between moving parts (FIGURE 7-1). Since diesel thermal efficiency is almost double that of gasoline engines, less heat from the combustion chamber is transferred to the cooling system. However, even though diesel engines transmit less heat to engine coolant, diesel engines have additional heat loads that are absorbed by the cooling system (FIGURE 7-2). These additional heat loads include: ■■

■■

FIGURE 7-1 This crankshaft journal was

damaged because of a lack of lubrication. The oil feed hole is plugged with bearing material, which melted during engine operation.

Cooled exhaust gas recirculation. Up to 30% of intake air mass in current diesel engines will include recirculated exhaust gases, which can reach 1,200°F (650°C). These temperatures are reduced to 400°F (204°C) or less before being mixed with fresh intake air. Lubrication oil. Since every turbocharged engine cools the underside of the piston with engine oil, hotter engine oil loses its viscosity, thus thinning too much to support loads and properly lubricate parts. Diesel engines will use oil coolers to reduce oil temperatures. Oil temperature sensors are often used to derate engine power or warn the operator if oil exceeds 250°F (121°C) (FIGURE 7-3). Air intake system. Intake air compressed by the turbocharger easily heats to over 400°F (204°C). Engines using series turbocharging will drive intake temperatures even higher than this. Engine coolant is often used to help reduce charge air temperatures. For example, the Powerstroke diesel of the 2010 model year and afterward use a liquid charge air coolers. A separate cooling circuit with its own water pump and radiator is used to lower the temperature of pressurized intake air.

2. The cooling system regulates temperature. If temperatures climb even as little as 12°F (5°C) above normal range (195°F–207°F or 91°C–97°C), cylinder wall wear begins to

FIGURE 7-2 Diesel cooling systems

absorb additional heat loads from components. High-capacity exhaust gas recirculation (EGR) coolers, air compressors, oil coolers, and turbochargers are just a few additional heat loads.

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Functions of a Cooling System

183

FIGURE 7-3 Oil cooler nozzles spray the

underside of all engine piston crowns with oil to remove heat. The heat absorbed by the oil is removed by the cooling system through the oil cooler.

dramatically increase. Conversely, cold cylinders will prevent engine parts from operating with optimal clearances, causing excessive oil consumption, carbon deposits on pistons, poor combustion quality, and oil sludging due to water condensation. In current diesel engines, the coolant must reach a minimum temperature necessary to enable onboard diagnostic (OBD) monitors or any emission control strategy to operate or execute within a reasonable time. In fact, an OBD cooling system monitor is dedicated to alerting the driver if the warm-up of the cooling system becomes irregular. 3. The cooling system provides for coolant expansion. As engine coolant warms, it expands. Cooling systems provide overflow reservoirs so that coolant can expand (FIGURE 7-4). 4. The cooling system pressurizes engine coolant. Cooling systems are pressurized for two important reasons. The first is to maintain a greater-than-atmospheric pressure at the water pump inlet. Without pressure to the water pump inlet, pumps operate less efficiently and can cause cavitation. Pressurizing coolant also raises its boiling point, thus preventing coolant from being lost due to boil-over (FIGURE 7-5).

0 (0)

2 (14)

FIGURE 7-4 Cooling system

components and coolant circulation patterns through a typical I6 heavy-duty diesel engine.

Radiator Cap Pressure psi (kPa) 6 14 16 8 10 18 12 (41) (55) (69) (83) (97) (110) (124)

4 (28)

290 (143) 280 (138) 270 (132)

50% Water/Ethylene Glycol

260 (127) Boiling Point °F (°C)

250 (121) 240 (116)

Plain Water

230 (110) 220 (104) 210 (99) 200 (93) 190 (88) 180 (82) 170 (77) 160 (77) 150 (66) 10 (69)

12 (83)

32 14 16 24 28 18 26 20 22 30 (97) (110) (124) (138) (152) (165) (179) (193) (207) (221) Absolute Pressure psi (kPa) Atmospheric Pressure

FIGURE 7-5 Increasing the pressure of

the cooling system increases the boiling point of the coolant. Adding antifreeze to the coolant further increases the boiling point of the coolant.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

1. Bubble forms.

2. Bubble starts to collapse.

3. Energy from the implosion of the bubble removes the oxide film from the cylinder wall. FIGURE 7-6 Hot coolant vaporizes, forming

a bubble on the side of the cylinder wall. The force of a collapsing vapor bubble is great enough to remove material from the cylinder wall.

▶▶TECHNICIAN TIP Core plugs or frost plugs are used to seal passageways in cylinder blocks and head castings, which function to flush sand from casting after manufacturing. Although these plugs may be displaced by frozen coolant, they are not designed to prevent engine damage.

▶▶TECHNICIAN TIP Coolant in the cylinder of a diesel engine will prevent the engine from rotating because the cylinder has very little clearance volume and cannot compress coolant. This condition is called hydrostatic lock. Coolant may enter through a leaking cylinder head gasket, intake air systems, or a pinhole-sized opening in the cylinder caused by cavitation erosion. An engine turned in the correct direction of rotation will lock up if liquids are in the cylinder. To confirm whether the engine is hydrostatically locked, the engine when rotated in the opposite direction of correct rotation will push liquids out the exhaust valve and the engine will freely rotate in that direction only.

4. Erosion and pitting of the cylinder wall if not protected by coolant.

5. Complete failure of the cylinder wall and coolant leakage into the cylinder.

5. The cooling system deaerates coolant. Cooling systems must deaerate coolant to prevent pumps and coolant passageways in the cylinder head from becoming air bound. Without deareation, vapor from hot coolant collects into large steam pockets, preventing heat transfer. 6. The cooling system minimizes corrosion. When dissimilar metals come in contact with engine coolant, the contact causes electrochemical reactions. Some metals will be severely and quickly corroded through these chemical reactions. Diesel antifreeze contains a number of additives that minimize the electrochemical reactions that cause corrosion. 7. The cooling system provides freeze and boil protection. Using antifreeze, also referred to as coolant, provides freeze and boil protection. 8. The cooling system minimizes cavitation erosion. A condition unique to diesel engines is cylinder wall and liner cavitation. Cavitation erosion is caused by the collapse of tiny water vapor bubbles formed when coolant rapidly heats next to hot surfaces and/or when the coolant is subjected to a rapid drop in pressure. Liquid will vaporize in both of these situations, producing gas bubbles. When these vapor bubbles collapse, the pressures are as high as 60,000 psi (413,700 kPa). The force of implosion against cylinder walls, cylinder liners, water pump inlets, injector tubes, and other components easily blasts holes into the components (FIGURE 7-6, FIGURE 7-7, FIGURE 7-8). Cylinder walls or liners have an exaggerated pattern of cavitation erosion on the coolant side of the major thrust surfaces because the liner surface flexes and relaxes during the piston’s power stroke. This expansion–contraction vibration creates a temporary lowpressure area on the outside of the wall or liner, producing even more cavitation erosion. Vibration and Cylinder Liner Movement Cavitation Damage

FIGURE 7-7 Cavitation erosion is unique

to diesel engines because of higher cylinder pressures and longer stroke lengths. Movement of the cylinder wall due to high combustion pressure and the thrust angle of the connecting rod creates a momentary void between the coolant and the cylinder wall.

Vapor Bubbles

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Cooling System Components and Construction

185

Note, parent bore blocks are not immune to cavitation erosion. Chemically treating the cooling system is a preferred method for minimizing cavitation erosion. Both new and old formulations of antifreeze for diesel engines use nitrite as an additive to minimize cavitation erosion. Nitrite added to the coolant forms a thin, sacrificially protective film to prevent cavitation erosion. Vapor implosion will instead damage only the nitrite layer, which is replenished from the nitrite dissolved in the coolant. Depending on the formulation of antifreeze, coolant will use varying amounts of nitrite, which requires periodic testing during service inspections to maintain the correct level of nitrite.

▶▶ Cooling

System Components and Construction

Durability of automotive-type diesel engines depends on the efficiency of liquid cooling systems. Water mixed with antifreeze plus additives to protect the cooling system make up the cooling medium used to absorb engine heat. A heat exchanger or radiator transfers heat from coolant to the atmosphere. The diesel cooling system includes several unique features, to be discussed in this section (FIGURE 7-9).

Coolant Diesel engine coolant, while similar in some respects to gasoline-fueled engine coolant, is not the same. Diesel coolant consists of water and antifreeze, and contains a specialized set of corrosion inhibitors referred to as supplemental coolant additives (SCA) or diesel coolant additive (DCA).

Water Water is the most efficient fluid used to transfer heat. However, water will cause metal to corrode when in contact with the cooling system, so it needs an additional corrosion inhibitor. Antifreeze should be added to a minimum of 40% concentration because of the corrosion inhibitors it contains. Too high a concentration of antifreeze, however, will reduce the ability of coolant to absorb and release heat. Results comparing the specific heat values of antifreeze water mixtures show it absorbs and releases only 50% to 70% as much heat as pure water. Poor engine cooling, overheating, and inadequate passenger warm air heating results when antifreeze is overconcentrated. For this reason no more than a 60% concentration of antifreeze is permissible, and more than a 65% concentration can also cause the coolant to lose its freeze protection. In cold climates, a 50/50 mixture of ethylene glycol–type antifreeze is recommended (ethylene glycol can be abbreviated as EG). Propylene glycol–based antifreeze requires a higher concentration to achieve the same freeze protection as EG antifreeze (propylene glycol can be abbreviated as PG). For boil protection, antifreeze concentrations in engines operating above 195°F should not be less than 50% (TABLE 7-1).

FIGURE 7-8 An injector tube damaged

by cavitation erosion. 7-02 Identify and describe the construction and operation of cooling system components.

SCA

Water (H2O)

Antifreeze

TABLE 7-1 Freezing Points of EG and PG Antifreeze Concentration of Antifreeze by Percentage of Volume

Freeze Point of Coolant EG

0% (water only)

32°F

0°C

32°F

0°C

20%

16°F

−0°C

19°F

−7°C

30%

4°F

−16°C

10°F

−12°C

40%

−12°F

−24°C

−6°F

−21°C

50%

−34°F

−37°C

−27°F

−33°C

60%

−62°F

−52°C

−56°F

−49°C

80%

−57°F

−49°C

−71°F

−57°C

100%

−5°F

−22°C

−76°F

−60°C

FIGURE 7-9 Diesel engine coolant

is made up of water, antifreeze, and supplemental coolant additives (SCA)— also called diesel coolant additive (DCA); SCA and DCA contain nitrite.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

Ethylene Glycol Antifreeze

▶▶TECHNICIAN TIP EG, which is the base used for the majority of antifreezes, is toxic. Less than 4 oz (113 g) will kill a 150 lb (68 kg) person. Because of its toxicity, EG is an environmental hazard; therefore, old antifreeze must be properly disposed of by a licensed recycler. Service garages are required by law to separate and store old antifreeze in specialized containers while waiting for recycling.

▶▶TECHNICIAN TIP Important criteria for selecting antifreeze includes whether the formulation is compatible with gaskets, coolant hoses, and other cooling system materials. Using the wrong type of antifreeze for a cooling system can slowly or quickly lead to deteriorated hoses and engine gaskets leaking. And cooling system materials can become corroded when the wrong type of antifreeze is used.

▶▶TECHNICIAN TIP Diesel engine and gasoline engine Type I “green” antifreezes are different and should never be interchanged. Gasoline engines use Type I antifreeze, containing higher silicate levels. Only low-silicate antifreeze should be used in diesel engines, since the addition of nitrite to diesel coolant will cause silicate dropout. Dropout or the appearance of green “goo” can lead to plugged radiators, water pump seal leaks, and an engine overheating.

Coolant has evolved considerably since the early days of automotive technology. At one time, coolant included mixtures of alcohol and water, as well as water mixed with honey and soluble oils. In the 1940s, EG was developed to provide a combined freeze and boil-over protection. When initially manufactured, EG is a clear, syrup-like fluid. However, green dye was initially added for identification since it is extremely poisonous. The majority of antifreezes currently manufactured use EG as a base. Today, EG-based antifreeze is colored with other dyes than green to designate specific formulations of corrosion inhibitors blended with the EG.

Propylene Glycol PG is made available for diesel engine cooling systems to minimize the problems associated with the toxicity of EG, an environmentally friendly antifreeze base. PG is an ingredient in many cosmetics and foods (e.g., hand cream, ice cream, milkshakes). It has slightly different physical properties than EG does. Since the density and concentration required for freeze protection is different from EG, more PG is mixed with water to obtain adequate freeze protection (FIGURE 7-10, TABLE 7-2).

Antifreeze Formulations The mixing of EG or PG antifreeze and water alone is not effective to prevent cooling system components from corroding or eroding. When exposed to air, EG will become acidic; PG, on the other hand, is corrosive to certain metals. Antifreeze formulation develops as an evolutionary process driven by the need to prevent corrosion, ensure material compatibilities, and minimize maintenance. While the bases of all modern antifreezes are PG or EG, the additive packages vary considerably, which can lead to confusion and severe maintenance problems if not properly understood and the correct formulation used. At least six different types of coolant formulations are used today. The additive package added to the antifreeze base differentiates between the various types of EG- and PG-based coolants. Coolants are usually dyed according to The Maintenance Council (TMC) recommendation to help distinguish between the various formulations. Note, however, that some coolant manufacturers do not follow TMC recommendations.

Recharge Fair

Good

—84 (—64)—70 1.400 —70 (—57)—64 —60 (—51)—50 1.350 —50 (—46)—56 —40 (—40)—52 1.300 —30 (—34)—48 —20 (—29)—44 1.250 —10 (—23)—39 —5 (—21)—36 1.200 1.150

1.100 Battery Fluid

0 (—18)—33 5 (—15)—29 10 (—12)—25 15 (—9)—21 20 (—6)—16 25 (—4)—10 Ethylene Glycol

—60 (—51)—63

—50 (—46)—60 —40 (—40)—57 —30 (—34)—53 —20 (—29)—49 —10 (—23)—44 —5 (—21)—41 0 (—18)—38 5 (—15)—34 10 (—12)—30 15 (—9)—25 20 (—6)—19 25 (—4)—12 Propylene Glycol

32 (0%) 32 (0%) °F (°C)—% glycol by volume

FIGURE 7-10 A refractometer

screen has two different scales for measuring the freeze protection of PG and EG coolants.

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187

TABLE 7-2 Antifreeze Types Antifreeze Type

Performance Specification

Suggested Color

Type I conventional low silicate using inorganic acid technology (IAT)

Technology & Maintenance Council (TMC) RP-302a

Green

Type II fully formulated EG extended life using organic acid technology (OAT)

Technology & Maintenance Council (TMC) RP-3298

Purple or pink

Type III fully formulated PG extended life using OAT

Technology & Maintenance Council (TMC) RP-330

Blue

Type IV OAT

Technology & Maintenance Council (TMC) RP-338

Red

Hybrid organic acid technology (HOAT)

Combination of IAT and OAT with nitrites added

Ford—yellow; Daimler-Chrysler—orange

Nitrated organic acid technology (NOAT)

Similar to HOAT

No color designated

Type I: Conventional Low-Silicate Antifreeze Antifreeze contains corrosion inhibitors to protect cooling system materials such as cast iron, aluminum, copper, brass, solder, steel, and many nonmetallics such as nylon and silicone. Type I conventional antifreeze uses older additive technology and contains corrosion inhibitors such as borate, phosphate, and sodium silicate. These types of inhibitors are known as inorganic additive technology (IAT) and are identified by a green dye. Sodium silicate is used primarily to protect aluminum and is found in a much higher concentration when used in gasoline engines that have aluminum cylinder heads. A problem with sodium silicate in diesel engines, however, is the incompatibility of silicate with nitrite, which is added to protect against cylinder wall cavitation erosion. High concentrations of both these additives together will cause a condition known as silicate dropout. Dropout has a characteristic appearance of green “goo” or gel, which plugs radiator and heater core tubes, causing the engine to overheat and even catastrophic failure (FIGURE 7-11). For this reason, Type I antifreeze is blended differently for diesel engines than it is for gasoline engines. Foam suppressants are added to maintain coolant contact with engine parts since steam and other vapor bubbles do not transfer heat well. Type I additives deplete quickly and need to be monitored. Additives, such as phosphorous, drop out with silicate and coat cooling system components with a layer of slime-like deposits. It is important to drain and replace Type I antifreeze that contains this corrosion inhibitor package every two years (TABLE 7-3). To maintain the correct level of additives, the concentration of nitrite in Type I coolant needs to be tested, typically using test strips, to ensure the correct levels of nitrite and other inhibitors are present. Between 3% and 5% of DCA or SCA are added after the initial mixing of antifreeze and water to bring the additive package for diesel engines to the correct protection level (FIGURE 7-12). Monitoring the additive package is necessary every time the engine is serviced, to maintain the correct levels of the various corrosion inhibitors.

Silicate Contamination

Partial Radiator Tube Blockage FIGURE 7-11 Silicate dropout forms

a green gooey substance in the cooling system that blocks coolant tubes in radiators and heater cores.

TABLE 7-3 Type I Antifreeze Inhibitor Additives Type I Antifreeze Inhibitor

Protects

Phosphate

Iron pH control (keeps coolant alkaline between 8.5 and 10.5 pH)

Borate

Iron pH control (keeps coolant alkaline)

Sodium Silicate

Aluminum

Nitrite

Cast iron Steel Aluminum Solder

Mercaptobenzothiazole (MBT) and Copper Tolytriazole (TT) Brass

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Extended-Life Antifreeze Given the high maintenance requirements of Type I antifreeze and its short two-year service interval, manufacturers have sought to develop new coolant technology to minimize cooling system maintenance and extend the service interval, called extended-life coolant (ELC). To achieve this, a new category of antifreezes has been developed with corrosion inhibitors made from organic acids. Organic acids are different from inorganic acids in that they contain carbon. Referred to as organic acid technology (OAT), these inhibitors are generally low or free of nitrite, nitrate, phosphate, silicate, and borate (TABLE 7-4). OAT antifreeze has the advantage of being more chemically stable, which allows for exceptionally long service intervals. The formulation provides maximum protection of the six basic metal alloys found in most cooling systems. Since the coolant generally contains no phosphates or silicates, deposits in the cooling system are almost eliminated. The low level of abrasive solids dissolved in antifreeze results in improved water pump seal life. Antifreezes using this coolant chemistry are categorized also as long life, FIGURE 7-12 This test strip checks the nitrite levels in OAT, hybrid organic technology, and nitrited organic acid technology. Variaeither Type I or extended-life coolant. A different scale on tions in the inhibitor package differentiate between these types of antifreezes. the side of the bottle is used for each type of antifreeze. To maintain these fully formulated antifreezes at every preventive maintenance inspection or at least twice per year, the color and freeze point and nitrite level of the coolant is checked.

Type III: Low Toxicity Antifreeze

▶▶TECHNICIAN TIP Sensors and other electrical devices exposed to engine coolant have the potential to short out through the coolant. Electrical current passing though the coolant will quickly corrode brass and aluminum oil coolers, injector tubes, and other metals in the cooling system. To minimize this problem, engine blocks use a number of ground straps not only to lower resistances for engine-mounted electrical devices but also to minimize electrical conduction through engine coolant. Electrical conduction through coolant quickly eliminates nitrite additives in coolant, leading to further engine damage.

Type III antifreeze is used mostly in off-road applications due to its low toxicity. In the event of a coolant leak in off-road situations, PG’s low toxicity will not contaminate soil to the extent that EG would (FIGURE 7-13). Type IV: OAT Antifreeze. The OAT-type antifreeze uses non-carboxylate acids, such as benzoate, from benzoic acid, to form the additive package. This red antifreeze is commonly used in today’s light- and heavy-duty diesel and gasoline engine.

Hybrid Organic Acid Technology HOAT is a combination of IAT and OAT with nitrites added. This makes HOAT suitable for use in both light- and heavy-duty systems. Versions of this antifreeze are dyed between orange and a straw-yellow color. This antifreeze is often marketed as universal antifreeze.

Nitrated Organic Acid Technology NOAT antifreeze is similar to HOAT. NOAT is an OAT with nitrates added. This chemical combination makes it suitable for use in both light- and medium-duty diesel cooling systems and is often marketed as a universal antifreeze. NOAT and HOAT are very similar in performance characteristics. Mixing IAT with OAT, HOAT, or NOAT antifreeze will not damage the engine’s cooling system, but it will nullify the extended-life characteristics of these formulations. Depending on the type of formulation, acceptable contamination

TABLE 7-4 Type II Antifreeze Additives and Their Purposes Type II Antifreeze Inhibitor

Protects

Potassium Soap of Dibasic Carboxylic Acid

Iron Solder Aluminum

Potassium Soap of Monobasic Carboxylic Acid

Aluminum Iron

Nitrite

Cast iron Steel

Molybdate

Iron

Tolytriazole

Copper

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can range from 10% to 25%. Because NOAT can easily be confused with the variety and complexity of antifreeze formulations available for use, the technician and diesel engine owner must be fully aware of what the vehicle manufacturers’ requirements are for antifreeze and follow those recommendations carefully. Since ELC additives can deplete with time, bottled SCA additives, called extender, which are specifically formulated for different types of OAT antifreeze, can be added to lengthen the coolant change interval by six to eight years.

Radiators Radiators are heat-exchanging devices that release heat absorbed from the engine to the air. Coolant can flow through the radi- FIGURE 7-13 PG is a nontoxic alternative to EG-based antifreeze. ator in two ways. In down flow radiators, the coolant will enter through a hose connected to the top tank that is carrying hot coolant from the water pump outlet. The coolant will then flow from the top tank, through cooling tubes, and into the bottom tank. Alternatively, in cross flow radiators, coolant will flow from a top-connected hose, through a side-mounted tank, into cooling tubes, and across the radiator to another side-mounted tank before reaching a bottom-mounted hose (FIGURE 7-14). Cross flow radiators are preferred in vehicles with low-profile aerodynamic hoods because they can be made wider with a lower height profile. Tubes inside the radiator core provide the surface area to exchange heat. Heat dissipates from the coolant through the tube wall and then through the fins. Air passing through the fins carries away heat, thereby allowing tubes and fins to absorb more heat from the coolant (FIGURE 7-15). Early radiator construction used copper brass radiator components. Copper brass construction seemed the obvious choice for the radiators because of its superior heat conductivity, how easy it is to form, and how easy it is to repair. However, it was learned that increasing

Radiator Cap

Radiator Cap Co

ola

nt F

low

Tubes

Fins

Oil Cooler

nt F

low

Oil Cooler

FIGURE 7-14 A. A down flow radiator. B. A cross flow radiator.

Coolant Flow Tubes Fins Air Flow

FIGURE 7-15 A tube-and-fin construction of a radiator.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

Coolant Recovery Tank

Hot Cold Pressure Cap

Coolant

Radiator (or surge tank)

FIGURE 7-16 When coolant is heated, it expands. The

the diameter of tubes in a radiator made radiators more efficient. This required thicker tube walls to provide mechanical strength but radiator construction techniques using copper and brass prevented its use in improved radiator designs. Copper is a good heat conductor but the solder required to bond the tubes to the fins creates an insulation point that prevents some heat transfer. Another problem is all the mechanical stresses are borne by the solder in the joints between the radiator tank and the tube core. Solder can be attacked if the coolant pH level becomes too alkaline through the excessive use of DCA/SCA. Today, the preferred method to construct a radiator core is to use aluminum tubes, which transfer heat well. Aluminum tubes are welded or crimped rather than soldered to the aluminum or plastic tanks. This type of joint provides a more efficient conductor for transferring.

Surge Tanks

Additional volume is necessary for the cooling system coolant so that the coolant can expand. Surge or overflow tanks provide the space for coolant and vapor to move into when the engine coolant is hot. When the engine cools, the stored overflow volume returns to the radiator. Sometimes, large radiator top tanks will perform the same function as a separate reservoir. Overflow tanks use a line connected to the radiator just below the pressure cap, allowing coolant to move back and forth during the cooling system’s thermal cycles (FIGURE 7-16). Surge and overflow tanks provide a coolant reserve for the gradual loss of coolant. Most of the leakage originates from radiator hose clamps. Constant torque spring clamps are the best clamps to use to minimize coolant loss, because they change dimensionally with the expansion and contraction of hoses and connections.

overflow reservoir or surge tank collects coolant leaving the cooling system through the overflow pipe below the neck of the radiator cap. After the engine cools, coolant is drawn back into the cooling system.

SAFETY TIP Never open the radiator cap of a hot engine: the coolant will be depressurized and boil over. The sudden conversion of hot water into steam pressure in the engine will cause water to explosively erupt from the radiator with projectile force. Anyone near the radiator can be scalded.

Radiator Caps Whereas early radiator caps simply prevented coolant from spilling out of the cooling system, today’s radiator caps have several additional functions. First, the calibrated pressure valve inside the radiator cap increases the pressure of the cooling system to raise the boiling point of coolant. This feature prevents coolant loss from boil-over (FIGURE 7-17). Second, a vacuum valve incorporated into the cap allows air or coolant to reenter the cooling system when the pressure drops after the engine cools (FIGURE 7-18). Faulty Vacuum Valve Operation

Vacuum Valve

Overflow Tube Coolant Flows from Recovery Tank Radiator Coolant

Pressure Valve Operation

Pressure Spring Overflow Tube Coolant Flows to Recovery Tank Radiator Coolant

FIGURE 7-17 An example of the pressure rating of a

radiator cap at which the pressure-relief valve opens is stamped on the cap: 15 psi (105 kPa).

Pressure Gasket

FIGURE 7-18 Operation of the pressure-relief function and vacuum valve

operation of a radiator cap.

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191

vacuum valves are often identified when the upper radiator hose collapses after the engine cools or when coolant is lost. A coiled spring inside the lower radiator hose, or a solid metal tube, is required to prevent it from collapsing during high engine speeds, when accelerating due to negative pressure at the water pump inlet. The radiator cap is also important for water pump efficiency. When the cooling system is pressurized, the pump operates more efficiently than the 85% efficiency without pressure cap. Finally, pressurizing the cooling system minimizes the degree to which the entire cooling system is cavitated, because vapor bubbles caused by low pressure in the cooling system are less likely to form.

Water Pumps To ensure that the coolant is positively circulated, an engine-driven water pump moves coolant through the cooling system. Pumps can be belt- or gear-driven. Although a geardriven pump does not rely on a belt, which minimizes service requirements and is more reliable than a belt for long distances, if the weep hole fails to drain coolant leaking past a defective water pump shaft seal, a gear driven pump is more likely to allow coolant to enter the engine. All water pumps are centrifugal turbine pumps, which means they become more efficient the faster they turn. The inlet to the water pump is the lower radiator hose or some point near the lower radiator hose. Water drawn into the pump is discharged into the engine. Under fast acceleration, internal engine block pressures between the pump and thermostat can reach as high as 40 psi (276 kPa). To reduce energy losses from driving water pumps, manufacturers are introducing electrically controlled variable speed water pumps to help reduce fuel consumption to meet greenhouse gas (GHG) targets (FIGURE 7-19). A clutch that controls the speed of the water pump is signaled by the electronic control module (ECM) by using a pulse-width–modulated (PWM) electrical signal. A speed sensor on the pump provides data for closed-loop feedback control of the water pump speed. The impeller, which transfers the pump’s energy to the coolant, is located in the coolant and may be made of plastic or metal (FIGURE 7-20). Impellers can wear if a significant amount of abrasive circulates through the cooling system. Impellers can also become separated from the shaft and cause poor coolant flow. This problem is usually identified when the engine overheats but the lower radiator hose, which is normally hot to the touch, is instead cold or lukewarm because radiator coolant is not being pulled by the pump into the engine. A special seal separates coolant from the bearings supporting the water pump shaft (FIGURE 7-21). If coolant reaches the bearings, the water pump will suffer a major failure. To prevent this from happening, manufacturers add a weep hole between the seal and the bearing (FIGURE 7-22). If the seal is worn and damaged, the weep hole allows coolant to leak from the seal cavity inside the water pump to prevent bearing damage. Some coolant leakage from a weep hole is normal, and some water pumps hold any small leakage of coolant in a small reservoir, where it evaporates before leaking out.

Cover

Intermediate Disc Belt Pulley

Anchor/Valve Lever Secondary Disc

Speed Sensor Bearing and Shaft

Electrical Coil and Connector Base Body

Pump Housing Impeller

Seal

FIGURE 7-19 An electronically controlled, variable speed water pump reduces engine drag

when coolant flow is minimal.

FIGURE 7-20 A centrifugal metal water

pump impeller.

▶▶TECHNICIAN TIP A coolant leak from the weep hole of a water pump indicates that the water pump seal is leaking. The weep hole prevents coolant reaching the water pump shaft’s support bearing. If coolant were to reach the bearing, a catastrophic bearing failure could occur. On gear-driven pumps, the weep hole prevents coolant from entering the engine and mixing with lubrication oil. When inspecting an engine for leaks, or when pressure testing a cooling system, keep in mind that a small amount of coolant leakage from a pump is acceptable, because the seal cannot form a perfect liquid-tight barrier. To pressure test the cooling system and check for leaks, follow the steps in SKILL DRILL 7-1.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

O-Ring Gasket

Impeller Shaft Bearing (Two Ball Bearings) Bearing Seal

Weep Hole

FIGURE 7-21 The water pump seal prevents coolant from contacting

FIGURE 7-22 The water’s pump weep hole on a 6.5 L GM diesel.

the pump shaft bearings.

Some weeping from the hole is permissible but should not drip when the cooling system is pressurized.

▶▶TECHNICIAN TIP Silicate, phosphates, and borates used as corrosion inhibitors are dissolved solids. In time, they crystallize and fall out, or precipitate out, of the glycol and water solution. These crystals can enter the area between the water pump shaft and seal, acting as an abrasive and causing the seal to wear prematurely. The result is a coolant leak from the weep hole of the water pump.

Adding cold water to hot engine operating with low coolant will thermally shock the engine and damage ceramic or sintered metal water pump seals. Cold coolant added to a hot engine can also crack in all of the engine castings. If an engine cannot be left to cool down before adding water, it should be added only when the engine is running.

Thermostats Thermostats help accelerate engine warm-up and regulate the temperature of the engine coolant by modulating or controlling the volume of coolant circulated to the radiator. Accelerating warm-up is particularly important for direct-injection (DI) combustion chamber engines since much less heat is transferred to the cooling system than in spark-ignition (SI) engines. A prolonged warm-up period is required at idle speed, and the engine will not reach operating temperature unless some auxiliary device is used to help transfer more heat to the engine coolant. Exhaust back pressure regulators are one such device. Exhaust back pressure regulators restrict exhaust flow out of the cylinders at idle, causing combustion temperatures to increase and higher exhaust temperatures to linger at the exhaust ports of the engine. The coolant thermostat speeds up warm-up when the engine is below its normal operating temperature, by blocking coolant flow from the engine to the radiator. At a predetermined point—that is, 180°F (82°C)—the thermostat will begin to open, and it will continue to open further as the

SKILL DRILL 7-1 Pressure Testing the Cooling System and Checking for Leaks 1. With the engine off, completely fill the engine coolant level with premixed coolant. 2. With the engine off, connect the radiator pressure tester by replacing the pressure cap with the pressure testing adapter. Larger caps, which are found on heavy-duty cooling systems, or unique original equipment manufacturer (OEM) pressure cap designs may not fit with a standard pressure cap tester. An adapter may be required to seal the tool to the neck of the surge tank or radiator. Pressurize the system to 15–17 psi (103–117 kPa). 3. Observe the pressure readings for 10 minutes. During this time, visually inspect the cooling system hoses, all hose connections, auxiliary coolant heaters, cab heaters, EGR coolers, EGR valves, turbocharger coolant lines, the water pump weep hole, all Continued

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gasket sealing surfaces, and any other components that have coolant flow. 4. Check the engine oil level and verify that the oil has a normal color. Oil that looks milky likely has coolant leaking into it. 5. Inspect the seals on the pressure cap and pressure test the cap. The seals should be intact and in good condition without any cracks. The same pressure tester used for the engine’s cooling system is used to test the cap. An adapter is placed between the cap and test tool and the tester is pumped once again. The cap’s release pressure should be indicated on the cap, and it should hold pressure until the tester applies the amount of pressure specified on the cap. 6. Refer to the OEM service information to check whether the pressure cap is the correct pressure rating for the vehicle. If the cap will not hold or cannot reach specified pressure, or if it

193

does not vent at the specified pressure, then replace it with a new cap, one that has the correct pressure rating. 7. Record and report the results. 8. If there is a significant drop in pressure and the complaint investigated is coolant loss, the engine may have an internal coolant leak. The engine should be first checked to determine whether the cooling system is excessively pressurized during operation by using a separate procedure. The EGR cooler should also be inspected by using a separate procedure. 9. To check for internal leaks, drain the engine oil and remove the oil pan. 10. While the cooling system continues to be pressurized, observe whether coolant is internally leaking or if coolant is originating from a head gasket leak. In push tube engines, head gasket leaks will drip coolant from the cam side of the engine.

coolant temperature continues to increase: 5–10°F (2–8°C) later, the thermostat will be fully open (FIGURE 7-23). A variety of thermostat designs are commonly used in diesel engines. Choking-type thermostats are most commonly used in light- and medium-duty engines. These thermostats have a valve that opens and closes to regulate coolant flow through the engine; two thermostats are used in larger engines with higher coolant flow. Choking-type thermostats operate by using a special wax pellet combined with a powdered metal tightly squeezed into a copper cup that is equipped with a piston inside a rubber boot (FIGURE 7-24). Heat causes the wax pellet to expand, which pushes the piston upward, opening the valve (FIGURE 7-25). In engines not equipped with vent lines, a small jiggle or venting valve is located in a chocking thermostat to allow air and steam to escape past the thermostat and into the radiator (FIGURE 7-26). Faulty thermostats will cause the engine to overcool by staying partially open. An engine that will not warm up to operating temperatures, is very slow to warm up, or warms only under heavy load likely has a thermostat that is stuck partially open. Failsafe mechanisms Recovery Tank Hot Cold

Recovery Tank Hot

Recovery Line Pressure Cap

Deaeration Hose Top Hose Thermostat

Cold Deaeration Hose Top Hose Thermostat

Radiator

Fan Shroud

Fan

Engine

Water Pump

Bottom Hose Engine Cold

Recovery Line Pressure Cap

Bottom Hose Main Coolant Flow Deaeration Flow

Engine Hot

FIGURE 7-23 Coolant flow through a cooling system when using a full blocking thermostat.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems Frame Pin Seat

Valve

built into the wax pellet of contemporary thermostats prevent the thermostat from sticking closed, causing catastrophic engine damage. When overheated, the temperatures that the thermostat is exposed to will lock in the completely open position, but it still must be replaced because it cannot close again.

Fans and Fan Drives

Engine fans are designed to increase the airflow across the radiator, which improves the efficiency of heat removal from coolant. The number of blades, the fan speed, and the fan pitch will determine the volSpring ume of air moved by the fan. For most vehicles, engine fans pull air Copper Cup through the radiator in the same direction as airflow when traveling in a forward direction. When the vehicle is traveling, its movement Wax Pellet usually creates enough airflow, so the fan is not required. At low-speed FIGURE 7-24 Cross section of a choking thermostat. operation and at idle, air movement is often inadequate, and a fan is required for the cooling system and to move air across the air conditioning condenser and any other front-mounted heat exchangers. To reduce the parasitic loss of power from the engine, a fan drive mechanism is used to decouple or reduce the fan speed when airflow is not required. Rubber Diaphragm

Viscous Fan Clutches

Coolant cold, thermostat closed, flows back to engine (via water pump).

Bypass

One of several common methods of controlling fan operation is through a temperature-controlled modulating fan clutch. These fans are coupled to the drive through a highly viscous silicone fluid. The degree to which the clutch slips can be controlled by several mechanisms, including a temperature-sensitive bimetal control valve regulating the volume of silicone fluid in the clutch coupling. When the clutch is cool, little silicone fluid enters the clutch and the fan freewheels. As the temperature of air in the fan clutch area increases, more fluid enters the clutch mechanism, which creates more drag or silicone fluid shear within the coupling mechanism. Because the fan speed does not correspond to engine speed and is considered to be modulated or has a heat-dependent speed adjusted according to the heat load, it absorbs from air passing through the radiator (FIGURE 7-27). These clutches are quieter because fan speed is kept proportional to heat load. Because their operational speed is proportional to engine temperature, they conserve energy and extract significantly less energy from the engine compared to an engine with a directly coupled fan.

From Engine

To Radiator Coolant hot, thermostat open, flows to radiator.

From Engine FIGURE 7-25 Operation of a choking engine thermostat to

FIGURE 7-26 A jiggle valve in a choking thermostat enables trapped

regulate engine temperature.

steam to vent through a closed thermostat.

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195

FIGURE 7-27 This silicone cooling fan hub freewheels when the hub

is cool. As more heat is absorbed by the hub, internal drag causes the fan speed to increase closer to the driven speed of the hub. Note the bimetal spring in the center of the hub, which is connected to a valve that controls fluid movement.

FIGURE 7-28 An electrically controlled variable speed engine fan

reduces parasitic power loss and provides optimal ECM control of fan speed. The bimetal spring is replaced with an electrically controlled vavle.

Variable Speed Engine Fan To reduce parasitic loss caused by operating a cooling fan, many late-model engines use a variable speed cooling fan. The fan is controlled by the ECM by using a PWM signal to regulate the fan speed. An electrically operated control valve is used instead of the bimetallic heat-sensing spring in conventional viscous cooling fans. Slower fan speed also translates to less engine noise (FIGURE 7-28 and FIGURE 7-29).

▶▶ Construction

and Functions of a Lubrication System

The lubrication systems of diesel engines are designed to perform the following tasks: ■■ ■■ ■■ ■■

■■

■■ ■■

FIGURE 7-29 A variable speed engine fan connects the inner and

outer hub by shearing silicone fluid between discs. reduce friction between moving parts (FIGURE 7-30) cool internal engine parts such as pistons remove dirt, abrasives, and contaminants from inside the engine assist in sealing the combustion chamber by forming a film between the piston rings 7-03 Identify and describe the functions of a lubrication system. and the cylinder wall absorb shock loads between bearings and gears to protect engine parts while reducing engine noise store an adequate supply of oil minimize corrosion on internal engine components (FIGURE 7-31).

Emission legislation and durability requirements impose new demands on the lubrication system too. Oils now need to be compatible with the latest exhaust aftertreatment systems and biofuels while meeting customers’ expectations for longer oil drain intervals Direction of Movement

FIGURE 7-30 Oil lubrication

Direction of Movement

behaves like ball bearings, enabling metal surfaces to slide over one another without metal-to-metal contact.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

Turbocharger

CamShafts

Oil Return O

Vacuum Pump

Oil Return R Non-Return Valve

Oil Pressure Switch

Spray Nozzle (Piston Cooling)

Bypass Valve Oil Filter

Oil Pressure Control Valve

Crankshaft Oil Return

Oil Cooler Oil Return Non-Return Valve

Oil Pan

Oil Pump Oil Pressure Relief Valve Oil Level & Temperature Sensor FIGURE 7-31 Oil circuits and the placement of lubrication system components in an in-line four-cylinder turbocharged diesel engine.

to reduce operating costs. The following major lubrication system components function to meet these demands. ■■ ■■ ■■ ■■ ■■ ■■ ■■

engine oil oil pump oil pan oil cooler oil filter(s) oil-pressure regulating and pressure-relief valves oil level dipstick.

▶▶ Engine 7-04 Identify and explain the classification systems for engine oil according to oil properties and characteristics.

Oil Classification

Performance standards for oil are established by the American Petroleum Institute (API) and the Society of Automotive Engineers (SAE). The SAE establishes standards for measuring oil viscosity, called the SAE viscosity rating, and the API sets the remaining standards. To be approved for the operating conditions and in use protecting unique technologies of various manufacturers, engine oils classified by the API will carry the API designation, indicating their suitability for an engine application. For example, engines that use highspeed turbochargers and that recirculate exhaust gases into the combustion chamber will

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Engine Oil Classification

place different demands on oil than oil used to lubricate a naturally aspirated engine with little or no emission controls. Two distinct classifications of oils are recognized by the API. S series oils are designed to meet performance standards for spark igntion (SI) engines SI; C series oils are designated for use in compression-ignition engines. The C and S series oils are distinctly formulated due to the combustion by-products of gasoline and diesel fuel combustion. The API donut-shaped service symbol indicates which standards an oil meets. Engine oil carrying the API service symbol is certified only after specialized testing to determine whether it meets minimum API standards (FIGURE 7-32).

API Certification Mark for Diesels

197

Oil Performance Standard (as defined by API test criteria)

American Petroleum Institute Society of Automotive Engineers

Oil Viscosity (as defined by the SAE)

API Certification Mark

API certification marks for diesel engine oils meeting the API CK-4 and the new FA-4 standards are the latest standards for diesel oil FIGURE 7-32 1. The top half designates the oil’s performance introduced in 2016. CK-4 is designed for compatibility with bio- standard set by the API test criteria. 2. The center identifies the oil’s fuels and the latest exhaust aftertreatment systems. It is backward viscosity, which is a standard defined by the SAE. 3. The bottom compatible with CJ-4 and CI-4 oil. CJ-4 was the previous oil stan- half is the API certification mark. Separate API designations are dard, which was developed for engines to meet the 2010 model year used for gasoline engines and diesel engines. Gasoline engines may EPA (Environmental Protection Agency) Tier 5 bin 2 emission stan- carry a starburst symbol, whereas diesels do not. 4. The shaded dards. These engines typically used EGR diesel particulate filters FA-4 designation helps distinguish the oil from CK-4 oils. (DPF) and ultra-low-sulfur diesel (ULSD) fuel. Since combustion by-products enter the oil from cylinder blowby, engines that use ULSD and EGR need ▶▶TECHNICIAN TIP oils capable of holding more soot produced by the EGR containing less acidity derived Diesel engine oil turns darker more from burning fuel with almost no sulfur. By-products of oil burned in the cylinders will quickly than oils used in gasoline engines. end up in the particulate filter, where oil additives leave ash residues that must be physSoot produced during diesel combusically cleaned out of the filter. The 2010 and later oils produce minimal ash residues, tion will cling to the oil film on the cylwhich will block exhaust gas flow through a DPF and cannot be removed by regeneration. inder walls and is scraped into the oil Substances capable of deactivating catalyst material are removed from this category of oil. pan. Soot, which is composed primarily FA-4 oils are a new category for use in high-speed four-stroke cycle diesel engines of black carbon, turns engine oil black. designed to provide better engine protection and help meet the 2017 model year on-highNewer engines with finer fuel atomizaway GHG and Tier 4 non-road exhaust emission standards. The “F” in FA-4 designates tion and engines operating consistently at the oil as a fuel-efficient oil, which is expected to reduce fuel consumption by up to 1.6%. high load and speed conditions will take A future oil would be designated FB-4. The new oil represents a large divergence in oil much longer to turn dark since less soot classifications to date and is not necessarily backward compatible with any CJ-4 or CI-4 is produced. oils. Because these new oils have much lower viscosity (are much thinner) to help reduce parasitic power losses caused by pumping and shearing thicker oil, they could potentially ▶▶TECHNICIAN TIP damage an engine that needs heavier, more viscous oil film. The term “high-temperature, high-shear (HTHS) viscosity” refers to FA-4 oils’ energy consumption–reducing properties, Combustion by-products such as soot Soot Tolerance by Category

Relative Valve Train Wear

100%

CG-4 (1994)

80%

60%

CH-4 (1998)

40% CH-4+ (1999) 20%

CJ-4 Plus (2002) 2%

6% 4% % Soot in Lube Oil

10%

FIGURE 7-33 Diesel engine oils

are formulated to tolerate the increased quantity of soot from diesel combustion. Retarded injection timing and using EGR contribute to higher soot levels in diesel engine lube oil. Adding extra detergents in oils formulated for diesels helps keep the soot suspended in lubricating oil, where it can be removed by the oil filter. Without the detergents, soot would settle and cover internal engine parts and form heavy sludge capable of blocking oil passageways for the lubrication system.

and other residues stick to oil on the surface of the cylinder wall, where they are scraped into the oil pan. Using EGR can introduce more soot into the oil and requires oil to be better able to dissolve and hold soot in the lubricating oil until it can be removed by the filter. Extra detergents in engine oil cause soot to stay in suspension better than oils with little or no detergent. Note too that excessive idling and stop and start operations also contribute to higher levels of soot loading in engine oil. Soot-thickened oil contributes to accelerated engine wear since soot is an abrasive. Soot also combines with water to form sludge, and thickened oil increases engine cranking resistance when cold (FIGURE 7-33).

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

which help engines meet lower GHG targets for CO2 emissions. More precisely, CK-4 oils are referred to as high HTHS engine oils, and FA-4 oils are being referred to as low HTHS oils. These oils will operate only in engines with relatively new hardened bearing shell technology, running with extraordinarily smaller oil clearances, which maintain higher oil film strength. API FA-4 oils are also designed to operate in much hotter engine environments, which means that they need to be better able to prevent oil oxidation caused by high temperature, which leads to thickening. The oil can also minimize viscosity loss due to molecular shear—cutting up chains of oil molecules by bearings and shafts. Additional antifoaming agents prevent FA-4 oil aeration. And like previous oils, when consumed by the engine, the oils do not contain additives that can poison exhaust catalysts or block particulate filters.

Oil Viscosity High Viscosity

Low Viscosity

FIGURE 7-34 Viscosity is a liquid’s resistance to

flow. Low-viscosity oil flows more easily than highviscosity oil.

Cold Temperatures

The term “viscosity” refers to a liquid’s resistance to flow (FIGURE 7-34). A numerical designation developed by the SAE is used to measure oil viscosity. The numbers indicate whether a particular oil flows easily or slowly. For example, engine oils that have a viscosity with low numbers such as 0, 5, or 10 flow more easily than viscosities of 20, 30, or 50. In other words, the higher the number, the thicker or more viscous the oil. Oil viscosity decreases when hot. Likewise, oil thickens and the viscosity increases as its temperature decreases (FIGURE 7-35). Oil viscosity measured at 0°F (−18°C) is given a W for a suffix, which is short form for winter viscosity rating. Oil viscosity measured at 212°F (100°C) is referred to as a hot or summer viscosity and no letter accompanies the viscosity grade. Oils with a single number are called a straight grade or single grade (FIGURE 7-36). Oil viscosity is indicated by two numbers (e.g., 15W and 40 in 15W-40), which means viscosity is measured under both hot and cold conditions. Referred to as multigrade or multiweight viscosity oils, these oils can be a blend of several different oils with different viscosities or blended oil with viscosity-changing additives. The numbers provide viscosity information

Hot Temperatures 50

SAE Viscosity Rating

SAE 10W Single Grade

SAE 10W-30 Multigrade

SAE 50

20W SAE 20W/50 10W

SAE 20W

FIGURE 7-35

SAE 30 Single Grade

Temperature changes an oil’s viscosity. Multigrade oils have a lower viscosity index than single-weight oils. That means multigrade oil viscosity does not change as dramatically with temperature.

122 (50)

212 (100)

Temperature °F (°C) FIGURE 7-36 Temperature changes an oil’s viscosity. Additives

and blends of oils with different viscosities can alter the rate at which oils change viscosity with temperature.

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Engine Oil Classification

about the oil’s performance under a wide range of operating conditions. The low-temperature viscosity (the first number: e.g., 15W in 15W-40 oil) helps predict how well an engine might crank in cold temperatures when resistance to cranking is high due to thick oil. Generally, the lower number means the engine will start more easily in cold weather and flow more quickly to critical parts. The high-temperature viscosity (the second number: e.g., 40 in a 15W-40 oil) provides information about the oil’s thickness or body, for good lubrication and load-bearing characteristics at normal engine operating temperatures. The main advantage of multigrade oils is improved cold-starting characteristics with less engine drag.

Viscosity Index Change and the Viscosity Index Improver

199

High

Viscosity

High VI

Low VI

Low

Cold

Temperature

Hot

Looking at the viscosity numbers of multigrade oil—e.g., 15W-40. It FIGURE 7-37 The VI of oil refers to how much its viscosity appears that the oil is thicker when hot and thinner when cold. How- changes with temperature. A high VI changes little, whereas a ever, this grade of oil is still thicker when cold than when hot, because low VI changes considerably. Ideal oils have a high VI. a cold 15W winter viscosity scale measures viscosity differently than the summer scale. This means 40W is still thicker than a hot 40 weight and a 15W is more viscous than 15 hot. Another feature is built into multiweight oils to control viscosity change, called the viscosity index (VI) improver. The VI of an oil refers to how much the oil’s viscosity changes with temperature. The viscosity of an oil with a low VI does not change much with the temperature, in contrast to an oil with a high VI (FIGURE 7-37). The VI improver is a special VI Polymer Expands to Increase Viscosity hydrocarbon polymer or, more simply, a molecular chain that changes shape with temperature (FIGURE 7-38). When cold, VI improver polymer chains will HEAT coil into a ball-like shape. This causes the oil to be less viscous than oil without the VI improver molecule. When heated, the polymer chain stretches into a long rod shape. This action tends to thicken the oil, counteracting the natural tendency of the oil to thin. When added to oil, the VI improver will counteract VI Polymer Coil at Room Temperature the tendency of oil to thin when hot and thicken when cold. Because oil in diesel engines gets very hot taking on the job of cooling piston crowns, this viscosity adaptation feature is especially important. Eventually, VI additives wear out as the polymer chains are broken through COOL the mechanical action of shearing the oil molecules. Moving oil through pumps and between bearings at high pressure and scraping oil from cylinder walls literally chop or shear molecular chains of VI improver, causing oil to lose VI VI Polymer Contracts to Decrease Viscosity improver properties. Shearing VI improvers explains why oil levels remain initially stable for a few thousand miles but suddenly drops with extended use. When VI improvers are sheared, they can no longer thicken oil when hot, and then the oil thins out. Thinner, less viscous oil will pass around piston rings and FIGURE 7-38 The volume of a VI improver molecule valve guides more easily and burn (FIGURE 7-39). Viscosity Modifier

expands with temperature to thicken engine oil as its temperature increases.

Detergent Inhibitor Diesel Engine Oil

Base Stock

FIGURE 7-39 Relative values

of viscosity improvers and detergents in diesel engine oil.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

Common Oil Additives Additives are blended with diesel engine oil to help it meet API standards. The following are some of the common additives: ■■ ■■

■■

Pour point depressants help oil flow in cold conditions. Antiwear additives protect against metal-to-metal contact under high-pressure conditions, when oil viscosity is low. Detergents and dispersants are used to keep internal engine components clean and prevent the oil from soot thickening and sludging. Oxidation inhibitors are added to maintain oil viscosity stability between changes. (Oil reacted with oxygen in the air will thicken, particularly at higher temperatures, and form deposits.) Corrosion and rust inhibitors needed to protect engine parts against the effects of condensation and acid. Antifoaming additives—minimize oil foaming and the accompanying loss of system pressure due to oil becoming more elastic when pressurized. Emulsifiers are used to prevent water from combining with engine oil.

Reserve alkalinity additives are used to protect the engine from acidic oil caused by the products of combustion. When sulfur in diesel fuel combines with water vapor, they will produce acidic oil over an extended period of time. The total base number (TBN) is the measurement of a lubricant’s reserve alkalinity, which aids in the control of acids forming during the combustion process. The higher an oil’s TBN, the more effective it is at preventing the formation of wear-causing contaminants. The total acid number (TAN) refers to oils level of acidity, which can increase proportional to fuel consumption. TAN increases as TBN decreases.

Detergents and Diesel Engine Oil Diesel engine oil turns blacker more quickly than oil in gasoline engines since diesel combustion produces more soot. Soot sticks to the oil film in the cylinders and is scraped off the cylinder walls and into the crankcase by the rings. Diesel engines require more detergent and dispersant additives to prevent the soot from clumping and thickening the oil. Filters are able to remove soot from the oil when it stays in suspension. Using oil without detergents or inadequate level of detergents will allow oil to thicken and possibly block oil passageways to bearings (FIGURE 7-40 and FIGURE 7-41). In contemporary oil classification, the +4 designation to CI, CJ, or CK oils indicate the oil has high amounts of detergent.

FIGURE 7-40 Detergents in oil enable soot and other solids to stay in suspension

where the filter can remove them. Two types of filter media: cellulose—yellow paper; microglass—white media.

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Factors That Limit Engine Oil Life Spring Oil Oil Oil Inlet Outlet Inlet

201

Valve

Oil Outlet Oil Inlet

Spring Valve

Bypass Valve Inside Oil Filter

▶▶ Factors That

Bypass Valve inside Mounting Pad of Oil Filter

FIGURE 7-41 A filter bypass

mechanism will allow oil to flow around filter media that are plugged with soot and solids.

Limit Engine Oil Life

It’s in the interest of owners and the environment to extend oil change intervals. Owners are motivated to sample engine oil to determine whether any oil properties are depleted. Oil needs not be changed if oil properties are intact and extending oil change intervals lowers operating costs. Important clues about impending engine failures are detected before a catastrophic failure occurs. From an environmental viewpoint, extending oil change intervals reduces environmental waste. The EPA reports that of the 1 billion gallons of engine oil consumed every year, about 185 million gallons of it is disposed of improperly after is has been used. Likely the oil is dumped into the ground, tossed into landfills, or poured into sewers. The EPA now encourages vehicle owners to follow manufacturers’ oil change recommendations and not the oil change industry’s. Research has demonstrated oil changes can be extended by using higher quality oil and using oil analysis.

7-05 Identify and explain factors that limit oil service intervals.

Synthetic Oils Synthetic‑base lubricants have been used in aviation and special‑purpose engines since World War II. Tanks used in World War II needed synthetics to allow the oil to properly lubricate the engines in extreme cold. High temperatures encountered in jet engines would burn petroleum-based oils, necessitating the use of synthetics. Synthetic lubricants did not begin to make inroads into automotive applications, however, until the late 1970s. Synthetic oils generally refer to oils whose base stock is synthetically derived or manufactured. Most brands of synthetic oils are extracted from ultra-refined petroleum oil, and the most popular base stock for synthetic oil is made from a petroleum-derived oil molecule called polyalphaolefin (PAO). PAO molecules are smaller and more consistent in size, and no impurities are found in this oil, because it is derived through a chemical process, making it very chemically stable and flexible compared to conventional oil. Approximately 70% of the base stock of synthetic oils is made from PAO. Chemical additives are then blended with the synthetic oil base stock, similar to the process of making conventional oils. However, synthetic oils usually have more additives than conventional oils. No API or SAE standards exist yet for defining what synthetic oil is, but they do need to meet minimum API specifications. Synthetic oils are of tremendous benefit to high-speed turbocharged diesels for at least nine reasons: 1. Turbochargers and rocker shafts receive lubrication faster than petroleum-based lube oils do. Since synthetics flow easily at cold temperatures, engine components, particularly the turbocharger, do not starve for oil during cold start-up conditions.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

2. Synthetic oils are resistant to coking. Coking occurs when oil is burned and forms carbon deposits. In turbochargers during hot shutdown, exhaust heat in the turbine housing moves into the center bearing housing, which can cause the turbine shaft bearing to heat and coke oil. Shaft bearings are damaged and the turbine shaft scored because of the abrasive carbon residue left from coked oil. 3. Improved shear strength of more flexible molecules occurs. Engines that use unit injectors, for example, exert greater stress to oil through shearing. Synthetic oil viscosity remains stable over a longer period of time. 4. Synthetic oils have a lower VI index. They flow better at low temperatures and resist thickening at higher temperature. 5. Synthetic oils have a lower friction coefficient. Most high-quality synthetics reduce friction, improving fuel economy and Probe on Style Valve lowering the engine oil temperature. Remove Cap and Flush Valve First 6. Synthetic oils decrease oil consumption: When synthetic oil is used, 42% less oil consumption occurs. This is due to less Turbulent Fluid evaporation and due do its resistance to deposit formation. Leave ¼ of Line Bottle 7. Synthetic oils have a greater resistance to chemical breakdown, (Return Line) Unfilled oxidation, coking, and deposit formation. (Ullage) 8. Synthetic oils have a lower coefficient of friction (they are slipperier). Users of synthetic oil have claimed a 2–5% increase in their fuel economy. 9. Synthetic oils have longer intervals between oil changes due to their higher resistance to oxidation. Twist Tie Rod Measured Standoff

Vacuum Sampler Sample Inlet Sludge Line

Sample Tube Inserted into Sump via Dipstick Tube FIGURE 7-42 A clean representative sample of oil is removed for analysis.

During SOS, oil can be removed from the oil pan or engine oil gallery.

Scheduled Oil Sampling In the absence of any sensor capable of comprehensively measuring oil quality, scheduled oil sampling (SOS) is recommended. A sample of engine oil is obtained from the engine and sent out for analysis (FIGURE 7-42). SOS can provide an extremely detailed analysis of the condition of the engine and quality of the engine oil (TABLE 7-5).

TABLE 7-5 Measures of Oil Quality for Analysis Indication in Oil

Possible Cause

Viscosity increase

Soot loading Oxidation Water contamination

Viscosity decrease

Fuel dilution VI depletion from shear Overheated oil

Depleted additive

High operating temperature Extended oil change intervals Addition of aftermarket additives Incorrect oil

High solids content

Dirt contamination Excessive metal wear

Water contamination

Extended idle time Coolant leak

Antifreeze contamination

Head gasket leakage Cylinder liner O-ring leak

Fuel contamination

Injector damaged Cylinder misfire Injector O-ring damage

Metal contamination

Excessive wear or damage indicated by type of metal contaminant (i.e., lead from bearings, chrome from rings, tin from pistons, iron from gears or block material, silicon from coolant additive, etc.)

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Factors That Limit Engine Oil Life

203

Oil Quality Sensors and Level Sensors Oil quality sensors are now available to evaluate the condition of lubrication oil but are not mature in terms of technological sophistication to perform condition-based oil changes. Such sensors, however, can help to alert the operator to a situation in which oil quality falls outside specified parameters, which could be either due to the oil exceeding its useful life or a symptom of an engine problem. One of several sensor technologies can accurately measure soot content, which absorbs oil additives and contributes to engine wear. The sensor operates like a variable capacitor, using oil as the dielectric element. By measuring alternating current conductivity across the sensor at frequencies between 2 and 5 MHz, the sensor evaluates the dielectric strength of oil by measuring the time needed to charge the sensor’s capacitor plates. This variable will change in proportion to the soot loading in oil. Oil level sensors are used to alert the operator to low oil levels. One type can use ultrasonic sound waves to monitor the oil level in the oil pan during the brief initial key-on-engine-off period. The sensor monitors the correct engine oil level to avoid an overfill or a low oil level while driving or at the key-on. Typically, the sensor is mounted on the bottom of the oil pan and has a piezoelectric transducer that sends ultrasonic sound pulses into the oil. Wave reflection at the oil surface and then back to the sensor measures the time the echo is received from the oil level surface to calculate the distance and hence oil level (FIGURE 7-43 and FIGURE 7-44). To locate an oil leak, follow the steps in SKILL DRILL 7-2.

5Vref

Sump

ECM

Oil Level

Signal Return

Signal Input

ECM Ground

+V 5 0

Signal (variable frequency square waveform)

Thermistor Microprocessor

Ultrasonic Sensor

FIGURE 7-43 An oil level sensor uses ultrasonic sound waves that reflect back from the surface of the oil level in the

oil pan.

FIGURE 7-44 Location of an oil level

sensor and connector on a 6.6 L Duramax oil pan.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

SKILL DRILL 7-2 Locating an Oil Leak

Distance Between Oil Changes

1. Obtain the correct type of fluorescent tracing dye. There are dyes that are compatible only with oil, fuel, coolant, or refrigerant. Choose the correct type.

Optimum Temperature Range

–40 (–40)

32 (0)

104 (40)

176 (80)

248 (120)

Oil Life Algorithm Systems

320 (160)

392 (200)

464 (240)

Oil Temperature °F (°C) FIGURE 7-45 Engine oil life is influenced by its operating

temperature.

Oil Deterioration Caused by Driving Type Oxidation Stability

City

Acidity (TAN) Alkalinity (TBN)

Oxidation Stability

Highway

Driving Type

Viscosity Change

Acidity (TAN)

Viscosity Change

Trailer

Oxidation Stability Acidity (TAN) Alkalinity (TBN) Viscosity Change

25 50 75 Percentage Deterioration

100

FIGURE 7-46 Oil chemistry changes depend on the type of

driving cycle the vehicle encounters.

Software-based calculations to determine when to change oil use a formula or algorithm with input variables from the engine operating conditions. This means a computer continuously monitors engine operating conditions to determine when to change oil. Since oil degradation is associated with combustion events, logging combustion conditions can accurately determine when oil life has reached its maximum value. For example, when a cylinder fires, a small amount of oil exposed to combustion is destroyed. Combustion gases containing acids, unburned fuel, water, and other by-products leak past the piston rings and react with engine oil. Diesel engines tend to generate much more soot and acidic combustion blowby into the crankcase than gasoline engines do. It is also known that oil temperature exerts a strong influence on the deterioration of oil quality. As the oil temperature increases, the rate of oxidation accelerates, which can lead to the oil thickening. Turbochargers subject engine oils to high temperatures and make an engine more prone to form engine deposits. Colder oil temperatures increase the concentration of oil contaminants, such as fuel and water. Depending on the vehicle and engine type, each engine receives its own unique algorithm to determine engine oil life (FIGURE 7-45 and FIGURE 7-46). The following are some of the other primary factors used to determine the limits to oil life, divided into subsections.

Percentage of Solids

Alkalinity (TBN)

2. Bring the engine to operating temperature, and add the prescribed amount of dye to the engine oil as recommended by the dye manufacturer. 3. Road test the vehicle with the engine under load. When checking for a leak, it is important to get the engine oil to operating temperature. High oil pressure and hot oil will reveal leaks faster than cold, low-pressure oil. 4. After running the engine for 20–40 minutes, shut the engine off and examine the engine by using an ultraviolet (UV) light. Some kits use alligator clips connected to the vehicle’s battery to power compact 12-volt lights. The newest dyes reflect a bright yellow-green at a leak path when using the UV light. The use of glasses with yellow lenses further enhances the appearance of the dye, making even small leaks easier to detect in tight spots. 5. If it is difficult to verify the actual spot that’s leaking, clean the area with an aerosol solvent and recheck the area while the engine is running. 6. Record all observations, and make a repair recommendation based on the findings.

The percentage of solids refers to the total amount of particles that are suspended in the oil. Combustion soot, dirt, and oxidized oil are typically the major solids found in oil. The percentage of solids becomes a limiting factor to oil life when the particles interfere with the lubricating abilities of the oil. Engine manufacturers suggest that an oil change to remove dissolved solids should occur before the solids rise above 5% by weight; however, improvements in wear reduction are best achieved if the solids are kept below 2%. Detergents are added to the oil to prevent the solids from clumping and forming damaging deposits inside the engine. Like hand cleaner, these detergents surround the particles and suspend them in the oil

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Factors That Limit Engine Oil Life

until it is drained or the filter removes the particles. It is normal for high-detergent oil to appear dirty after a short time. Just like dishwashing soap cleans dishes and keeps dirt suspended in the water, so too do oil detergent additives hold solids in suspension. More solids are removed through filtering oil with detergents. Chemical dispersants, called emulsifiers, are also used to keep these particles in suspension throughout the oil. Synthetic oils will require change intervals just like conventional oils if the percentage of solids is the limiting factor to engine oil life.

Sulfur and Combustion Acids Sulfur acids are present in engine oil whenever there is sulfur in the diesel fuel and corrosive acids produced by using recirculated exhaust gas are in the cylinders. Since sulfur combines with water produced during combustion, highly corrosive acids are formed in the oil. To counteract this condition, oil manufacturers add alkali, or base substances, to the oil to neutralize the effect of sulfuric acids. The use of more alkaline oil properties by diesels distinguishes oils used in SI engines from compression-ignition engines. The TBN is the measure of the engine oil’s ability to neutralize acid.

Oxidized Oil Oil is oxidized when it is exposed to air and the oxygen joins the oil molecules. Conventional oils generally thicken and form deposits if they are highly oxidized. This process occurs more rapidly at high temperatures. The problem with oxidation is that oil loses some of its lubricating properties as it oxidizes, thus causing accelerated engine wear. Synthetic oils are at an advantage since they do not oxidize as fast as conventional oils do. This means that if oil oxidation due to high engine operating temperatures is the limiting factor on lube oil life, then using synthetic oil could extend the oil change interval.

Biodiesel and Engine Oil For years biodiesel has been considered as a lubricity additive to keep moving parts operating smoothly inside fuel-injection systems. Dilution of engine oil with biodiesel is another matter. Biodiesel actually promotes dilution of engine oil with fuel to a much greater extent than petroleum diesel fuel does. Dilution of oil with biodiesel primarily occurs when a post-combustion injection of fuel into the cylinders is used to regenerate the particulate trap or NOx adsorber. All the fuel is expected to vaporize in the cylinder and not combust until it reaches the exhaust catalysts. Heavier, less volatile fractions of biofuel, however, do not vaporize during post-combustion injection, and liquid biofuel clings to the cylinder walls. Since biodiesel has a higher distillation temperature and boiling point, it will not evaporate from engine oil and it tends to dilute the engine oil disproportionately to its blend ratio in the fuel. As biofuel passes by the pistons and accumulates in the crankcase, it changes oil properties. Once there, biofuel polymerizes (joins together to form long, complex-shaped molecules), producing sludge-thickened oil. Engine damage can result from oil passageways that get blocked by sludge. Deposits also form in the ring belt area, leading to ring stickage and increased crankcase blowby, which further aggravates oil becoming contaminated by increased levels of soot. Raw or refined vegetable oils produce the greatest harm, which may not become evident until a significant amount of damage has occurred over an extended period. Petroleum-based fuel does not produce these same effects at all, since it readily evaporates when the oil is heated. Biodiesel blends higher than B20 (20% bio source) cause a larger amount of unburned fuel to slip by the piston rings and condense in the lubricating oil. Engine oil change intervals may need to be shortened significantly if blends of biodiesel higher than B20 are used (FIGURE 7-47). Biodiesel also interferes with the action of zinc dialkyl dithiophosphate (ZDP), an antiwear additive. The chemical properties of biodiesel cause the fuel to compete with ZDP for surface area on metals, thus displacing ZDP on metal. And detergents do not work as well when oil has been diluted with biodiesel.

205

▶▶TECHNICIAN TIP Whenever gasoline has been mistakenly used in a diesel engine, it is critical to drain and flush the fuel system, change fuel filters, and change the engine oil. Misfiring cylinders and gasoline will wash into the oil and dilute it. Similarly, always change the engine oil after any major repair to an engine, because the oil can be contaminated with dirt, coolant, or fuel. Never use an air gun to blow contaminants away from inside an engine. The contaminants are usually blown into the oil. Use a vacuum cleaner to remove contaminants from an open engine. Always cover an engine that is under repair when between work shifts, to prevent parts from being contaminated and to prevent the lube oil from eventually being contaminated with dirt. Leave parts in bags or boxes until just prior to installation.

▶▶TECHNICIAN TIP Engine oil is supposed to remain chemically neutral to prevent internal engine corrosion. Combustion gases dissolving in engine oil can make oil corrosive. Sulfur content is one element that can contribute to this change. To prevent these, alkaline additives are added to diesel engine oil. The TBN is used to measure the engine oil’s acid neutralizing ability. The TAN is another measure of a lubricant’s pH factor.

Other Limiting Factors Engine problems such as coolant leaks into the oil from sources such as a leaking head require an oil change immediately after a repair and before the engine is run. A defective air filter that permits dirt to enter the engine, fuel leaking into the oil, wrong fuel (e.g., gasoline

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

contaminating diesel fuel), water in the oil, or a mechanical failure that produces internal debris are all circumstances that necessitate an oil change. Many engines have experienced catastrophic oil-related failures subsequent to seemingly minor repairs when the oil has not been changed.

▶▶

il Pumps and Pressure O Circuits

Oil pumps are used to pressurize the lubrication system of a diesel engine. They are positive displacement pumps, usually constructed with a closed gear or gerotor design. A gerotor pump is constructed of an inner rotor and an outer rotor. The inner rotor turns inside the outer rotor ring, which also turns. The inner rotor has one fewer lobe than the outer ring and is positioned off-center from the outer ring. As the lobes slide up and over one another, oil is drawn through a fixed passageway connected to the oil pan pickup tube. Oil is squeezed out into the main oil gallery through the outlet passageway behind the rotors (FIGURE 7-48). The external gear pump is another common configuration for an engine oil pump. This pump has two opposing gears: A drive shaft turns one gear, which in turn rotates the opposite gear. Oil is trapped and carried around the inside of the pump housing between the gear teeth. A seal is formed FIGURE 7-47 This oil filler cap from a 6.7 L Powerstroke diesel indicates that the engine oil must be biodiesel compatible and identifies between the gear teeth and the oil pump housing (FIGURE 7-49). The lube oil is supplied under pressure to moving parts the grade of oil required for the engine. through drilled passageways in the engine called oil galleries. Pressurized oil is supplied to most moving parts, for lubrication. However, cylinder walls 7-06 Identify and explain the are lubricated by oil thrown off of engine bearings and from oil cooler nozzles. Oil is drawn construction and operation of into the pump through a pickup tube and screen located in the oil pan sump (FIGURE 7-50). pressurized oil circuits.

Oil-Pressure Regulating and Pressure-Relief Valves Rotor Ring

Housing

Inner Rotor

Suction

Discharge

Because the oil pump can produce more oil flow than the engine can use, an oil-pressure regulating valve is used to control oil pressure. Oil pressure that is too high can blow gaskets on the spin-on oil filter and burst or make the oil filter swell into a pumpkin shape. Excessively high oil pressure can also erode engine bearings. Keeping the oil pressure regulated to between 20 and 40 psi (138 and 276 kPa) when the engine oil is warm can reduce

FIGURE 7-48 A gerotor oil pump.

Housing

Driven Gear

Suction

Discharge

Drive Gear FIGURE 7-50 The oil pickup tube and screen is located in the oil pan FIGURE 7-49 An external gear oil pump.

sump.

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Oil Pumps and Pressure Circuits

parasitic power loss that is caused by pumping more oil than is necessary to regulate oil at a higher pressure (FIGURE 7-51).

Main Oil Gallery Oil Filter

Energy-Saving Oil Pumps The oil pump is a common target for achieving energy reductions in today’s diesels. Many new engine designs use flow rate– controlled oil pumps or two-stage pumps that operate at either a high-pressure or low-pressure setting, depending on operating requirements. Data from the oil-pressure sensor and ECM calculations will electrically switch the oil pump between the two pressure stages, depending on engine load, speed, and oil temperature. Power consumption by the oil pump is reduced when driving in low-speed urban or off-road conditions when high oil pump pressure is not required. For example, late-model Volkswagen (VW) and Ford 3.2 L Powerstroke engines use a flow rate–controlled vane pump that regulates pump delivery volume by rotating an eccentric adjustable ring that surrounds the pump vanes. The space between the pump vanes is increased or decreased to change pump output volume. A hydraulic control valve rotates the ring against spring pressure to vary the space swept by the pump vanes (FIGURE 7-52).

207

Engine Bearings

Bypass Valve

Pressure Regulating Valve

Filtered Oil Unfiltered Oil Oil Pump

Oil Pan Pick-Up Strainer FIGURE 7-51 Operation of the oil-pressure regulating valve. When

oil pressure exceeds the calibrated spring tension of the regulator, the check ball will uncover a passageway, permitting excess oil to return to the sump.

Oil Coolers Oil coolers are heat exchangers used to remove excess heat from engine oil. Diesel engines use oil coolers since pistons and turbocharger center housings are cooled by lubricating oil. Without a cooler, engine oil can easily become overheated while absorbing heat from internal parts and turbochargers. Engine oil should never exceed 250°F (139°C). Many electronic engines use oil temperature sensors to shut down or derate engine power if oil temperatures exceed 250°F to 260°F (139°C to 143°C). Normal maximum engine oil temperature is maintained at 30°F to 40°F (16°C to 22°C) above the temperature of the coolant. This means an engine operating between 190°F and 205°F (between 91°C and 96°C) could have a normal oil temperature of as high as 220°F to 245°F (122°C to 136°C) under load. Plate-type and tube-type oil coolers are the most commonly used design. Generally, oil coolers have a thermostatically controlled inlet for either coolant or oil that will not allow oil cooling until the engine oil or coolant has reached operating To Engine Oil Circuit via Filter Volume Control Solenoid Valve

Volume Control Piston Volume Control Spring

Volume Adjustment Ring

Vane Rotor

FIGURE 7-52 Components of a variable displacement oil pump that Volume Adjustment Spring Suction

are used to save energy. The area of the delivery space above the pump vanes is mechanically varied in accordance with engine demand for oil pressure.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

temperature. This hastens the oil getting warmed up, which improves engine lubrication (FIGURE 7-53). The cooler may also be equipped with a pressure-relief valve that allows oil to bypass the cooler if it becomes plugged. The bypass valve will open if a substantial pressure drop occurs across the cooler, such as when it becomes plugged with sludge or oil contaminants. Generally, since most engines are designed to allow oil to pass through the cooler first before being filtered, dirt, gasket pieces, silicone, and other materials can restrict how well oil flows through the cooler (FIGURE 7-54).

Cartridge Versus Spin-On Filters Spin-on oil filters, introduced during the 1950s, made changing oil easier and less messy. Since the late 1980s, however, European and now North American engine manufacturers are reverting to the cartridge-style filter for several reasons, one being underhood space. Because space is at a premium in the engine comFIGURE 7-53 This plate-type oil cooler is located near the outlet of partment of today’s vehicle, the oil filter can be very difficult to the oil filter. locate and change. New-style cartridge filter housings are usually conveniently located on the top or side of the engine compartment, making them accessible ▶▶TECHNICIAN TIP from under the hood without needing to raise the vehicle. Cartridge-type filter housings are Oil filters are considered hazardous typically designed with a screw-on-type cap and a single sealing gasket. The housings are waste and must be disposed of accordventilated when opened, and then the oil flows out of the filter back to the sump through a ingly. Both national and state environseparate drain system. The messy spills associated with spin-on filters are minimized this mental laws regulate how used filters are way. Cartridge-type filters are less expensive than spin-on filters. It is reported that almost to be disposed. Usually the hot crushing 80% of the cost of a spin-on filter is in the steel canister and valves. With a cartridge filmethod is required for oil filters to septer, only the pleated media and a gasket are replaced during filter service. The second, but arate oil from the filter before disposal. most important, reason for the change from spin-on to cartridge filters is the problems of Fuel filters must be crushed and drained spin-on filter disposal. Disposing of the filter element with even the smallest quantity of oil before disposal. is not only environmentally unfriendly but prohibited by law in many jurisdictions in North America (FIGURE 7-55). To replace a spin-on oil filter, follow the steps in SKILL DRILL 7-3. To Engine Lubrication Oil Filter Oil Filter Bypass Valve

Oil Cooler Oil Cooler Bypass Valve

Oil Pump

Pressure Relief Valve

Oil Strainer Oil Pan FIGURE 7-54 Bypass valves in the lubrication system

prevent either over-pressurization of oil or low oil pressure caused by restricted coolers or filters.

FIGURE 7-55 Cartridge filters are quickly replacing spin-on filters, due

to environmental regulations, space and accessibility requirements.

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SKILL DRILL 7-3 Replacing a Spin-On Oil Filter

1. With the engine off, remove the filter by using a filter wrench. A large belt-type filter wrench assisted by a power bar can be used to remove a tight filter. Occasionally, chain-type belt wrenches are used to remove tight filters. To prevent hot oil from scalding hands when the filter is removed, the filter may

have a hole punched into the bottom to allow oil to drain before removing it. 2. Visually inspect the filter base for any damage or warpage. Ensure that the old filter gasket is removed. 3. Fill the new filter with oil. Put oil in the unfiltered outer holes of the cartridge (the center filter hole returns filtered oil to the engine). 4. Lubricate the filter gasket by using oil or a smear of grease. This will help prevent the gasket from sliding on the filter base and prevent the gasket from buckling when the filter is tightened. It is also necessary to lubricate the gasket so that it’s easy to remove during the next service. 5. Spin the new filter on hand tight. Note the arrows or markings at the top of the filter canister, which indicate how far the new filter should be tightened after snugging the filter to the base by using hand force only. Most filters are tightened a quarter or half turn beyond hand tight. Tightening the filter further could potentially distort the filter base, which is often made of aluminum. The filter can also be damaged and internal leak paths formed if it is overtightened. If the filter is too loose, it will leak. 6. If the engine is being started for the first time after an overhaul, the engine should be prelubed with pressurized oil through an oil gallery plug. As an additional precaution to prevent damage to bearings and other moving parts from oil starvation, disable the fuel system by disconnecting an ignition fuse or crank and cam sensor or by using a remote starter button. Crank, but do not start, the engine until oil pressure appears on the direct reading gauge in the instrument cluster. Electronic gauges can be read if only the cam and crank sensors are disconnected. 7. After the engine is started and oil pressure is stabilized, check for leaks. Reset any maintenance monitor to indicate when the next scheduled filter and oil service is required.

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Diesel engine heat loads include exhaust gas recirculation (EGR), lubrication oil, and the air intake system. Diesel coolant consists of water, antifreeze (which contains a special corrosion inhibitor package), and supplemental coolant additives (SCA) that minimize cavitation erosion. Water is the most efficient fluid to transfer heat; however, water will cause metal parts in contact with the cooling system to corrode and therefore needs corrosion inhibitors added to it. An important job of the cooling system is to raise engine coolant temperature quickly to keep engine component wear to a minimum. As engine coolant warms, it expands, and cooling systems must provide room for this expansion. Cooling systems are pressurized to maintain greater-thanatmospheric pressure at the water pump inlet and increase the boiling point of water.

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Cooling systems must deaerate coolant to prevent pumps and coolant passageways in the cylinder head from becoming air bound. Antifreeze contains a number of additives that minimize electrochemical reactions, and these additives prolong the durability of engine and cooling system components. Freeze and boil protection are both supplied to the engine by antifreeze; pressurizing the cooling system provides additional boiling protection. Cylinder wall and liner cavitation is a condition unique to diesel engines; chemically treating the cooling system is a preferred method for minimizing cavitation erosion. Most antifreezes are made with a base of either ethylene glycol (EG) or propylene glycol (PG). Radiators are heat-exchanging devices that release heat absorbed from the engine to the air.

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

While early radiator caps simply prevented coolant from spilling out of the cooling system, today’s radiator caps have additional functions, such as increasing the pressure of the cooling system to raise the boiling point of coolant and to allow air or coolant to reenter the cooling system when the pressure drops after the engine cools. Surge or overflow tanks provide space for coolant and vapor to move when the engine coolant is hot. Vent lines, located at the highest points on the engine and radiator, bleed any steam and air out of the engine and into the surge tank. To ensure that coolant is positively circulated, an enginedriven water pump is used to move coolant through the cooling system. The lubrication system reduces friction between moving parts in an engine and is essential for cooling, cleaning, sealing, and absorbing shock loads inside the engine. Lubrication systems in today’s diesels have new demands because of emission legislation and durability requirements. Oil works by forming a separating film between parts, which minimizes direct contact between moving parts, reducing friction and heat buildup caused by friction. Engine oils are classified by the American Petroleum Institute (API) and carry an API designation that indicates their suitability for an engine application. Viscosity is a liquid’s resistance to flow; a numerical designation developed by the Society of Automotive Engineering (SAE) is used to measure oil viscosity. Additives improve the original properties of the base stock oil; enhancing the performance of motor oil base stocks is necessary to adjust the performance of the oil to suit its intended application. The major lubrication system components function to meet increasing demands for compatibility and owners’ expectations: the oil pump, oil pan, oil cooler, oil filter(s), and oil-pressure regulating and pressure-relief valves. The oil pump can produce more oil flow than the engine can use; an oil-pressure regulating valve is used to control oil pressure. Oil coolers are heat exchangers used to remove excess heat from engine oil. The main factors that can limit oil life are the percentage of solids suspended in the oil, sulfur and combustion acids, and oxidized oil.

Key Terms aeration A condition in which excessive amounts of air or steam bubbles are dissolved in coolant, diminishing the coolant’s effectiveness. additives Chemicals that improve the original properties of the base stock oil. American Petroleum Institute (API) An organization that has developed specifications to define engine oil performance standards.

base stock The raw mineral processed from crude oil. cartridge filter A filter element that consists of only filter media unenclosed by a metal container. cavitation Erosion in cylinder block walls, heads, and liner sleeves as a result of the collapse of tiny water vapor bubbles formed when coolant vaporizes on hot cylinder wall surfaces. coolant extender An additive package that is used only with ELC, which is added at the midpoint of the coolant’s life. diesel coolant additive (DCA) An additive used to treat cooling systems to reduce the effects of cavitation erosion. ethylene glycol (EG) The base chemical from which the majority of antifreezes are made. extended-life coolant (ELC) Several types of long-life coolant formulations that contain an anticorrosion additive package that does not deplete; also known as long-life coolant (LLC). heat exchanger A system that transfers heat from coolant to the atmosphere. heat rejection The transfer of heat into the cooling system from the combustion chamber. hybrid organic acid technology (HOAT) A combination of IAT and OAT with nitrites added, making it suitable for use in both light- and heavy-duty systems. inorganic additive technology (IAT) A diesel engine cooling system conditioner containing non-carbon-based corrosion inhibitors, such as phosphates, borates, and silicate. multigrade oil A blend of a several different oils with different viscosities; also known as multiweight oil. nitrated organic acid technology (NOAT) An ELC that uses OAT with nitrates added. oil quality sensor An electrical device that measures the amount of soot loading in engine oil. organic acid technology (OAT) A category of ELC that contains carbon-based corrosion inhibitors. polyalphaolefin (PAO) An artificial base stock (synthetic) used in place of mineral oil. PAO molecules are smaller and more consistent in size, and no impurities are found in this oil, because it is derived through a chemical process. propylene glycol (PG) An antifreeze base that is nontoxic and environmentally friendly. scheduled oil sampling (SOS) An extremely detailed analysis of the condition of the engine and quality of the engine oil. Society of Automotive Engineers (SAE) viscosity ratings Oil performance criteria that indicates oil’s flow characteristics. spin-on filter Filter media enclosed in a metal can that is threaded onto a filter header. supplemental coolant additive (SCA) An additive used to treat cooling systems to reduce the effects of cavitation erosion and optimize cooling system performance. synthetic oil Oil made from base stock that is synthetically derived or manufactured.

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thermal efficiency The ability of an engine to convert the energy content of fuel into mechanical force. total acid number (TAN) The acidity of an oil. Certain contaminants cause engine oil to increase in acidity, which is measured by using the TAN index. total base number (TBN) The measurement of a lubricant’s reserve alkalinity, which aids in controlling acids forming during the combustion process. viscosity A measure of oil’s resistance to flow. viscosity index (VI) A measurement of the total amount of change in an oil’s viscosity due to temperature. viscosity index (VI) improver An additive that prevents oil from thickening when cold and thinning when hot.

Review Questions 1. Which series of engine oils is used in a diesel engine? a. S series. b. ASTM. c. C series. d. SAE. 2. What is the maximum percentage of soot loading that means engine oil needs to be changed? a. 2%. b. 5%. c. 10%. d. 20%. 3. Which of the following is the normal operating oil temperature of a diesel engine? a. 20°F to 30°F (5°C to 14°C) lower than coolant temperature. b. Close to cooling system operating temperature. c. 30°F to 40°F (−1°C to 4°C) above cooling system temperature. d. Approximately 250°F (121°C). 4. The 15W in 15W-40 engine oil stands for which of the following? a. Multiviscosity. b. Viscosity at 0°F (−18°C). c. Viscosity at 212°F (100°C). d. The thickness of the oil when cold. 5. Which of the following will likely cause excessively high oil pressures? a. A plugged oil filter. b. A defective oil filter bypass valve. c. A defective oil-pressure regulating valve. d. A defective pressure-relief valve. 6. Which of the following ingredients is added to diesel engine coolant to prevent cavitation? a. Nitrite. b. Silicate. c. Ethylene glycol (EG). d. Borate. 7. Which of the following is the most recent engine oil made for a diesel engine? a. SG-4. b. CJ-4.

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c. SJ. d. FA-4. 8. Diesel engine oil has which job of the following extra jobs to perform, which gasoline engines are not necessarily required to perform? a. High-speed lubrication. b. Cooling piston crowns. c. Absorbing shock loading that engine parts can incur. d. Sealing piston rings. 9. The presence of cavitation on the coolant side of the cylinder wall is most likely caused by __________. a. lubrication breakdown b. excessive cylinder pressures c. excessive cooling system pressure d. improperly conditioned coolant 10. Which antifreeze will require the highest concentration to maintain 0°F (−18°C) freeze protection? a. Ethylene glycol (EG). b. Propylene glycol (PG).

ASE Technician A/Technician B–Style Questions 1. Technician A says that blue antifreeze is long-life antifreeze. Technician B says that blue antifreeze is propylene glycol–based antifreeze. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that a drop in the coolant level in the surge tank or coolant overflow reservoir when the engine cools indicates an internal engine leak. Technician B says that the drop in the level is normal. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that the cooling system pressurizes to enable coolant to remove more heat from the engine. Technician B says pressurization is needed to keep the coolant from boiling. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that antifreeze is added to coolant to protect the coolant from freezing and to lower its boiling point. Technician B says antifreeze lowers the freeze point and raises the boiling point of coolant. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says the problem with new long-life antifreeze is it can produce silicate dropout, which plugs cooling system passages. Technician B says that the problem

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Chapter 7 Diesel Engine Cooling and Lubrication Systems

with some types of long-life antifreeze is that it is not compatible with all cooling system materials. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says that low-sulfur oil needs to be used today to prevent damage to exhaust aftertreatment systems. Technician B says that today’s oils should have low ash content. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that viscosity index (VI) improvers thicken oil when it becomes hot and thins it when it becomes cold. Technician B says that VI improvers thin oil when it’s hot and thicken it when it’s cold. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 8. Technician A says that diesel engine oil turns black faster than oil in an SI engine because the oil composition and quality are different between the two oils. Technician B

says that there is no difference between diesel engine and SI engine oil. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says the oil-pressure regulator automatically reduces oil pressure at low-speed and low-load conditions. Technician B says that two-stage pressure regulation is performed electronically to reduce fuel consumption. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that spin-on oil filters in metal cans are being eliminated because they are less convenient and messy. Technician B says that cartridge-type filters are becoming more commonly used to allow them to be more easily disposed in a more environmentally friendly way. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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MASTER AUTOMOTIVE TECHNICIAN SERIES

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Light Vehicle Diesel Engines Gus Wright Light Vehicle Diesel Engines, published as part of the CDX Master Automotive Technician Series, prepares students with practical, accessible information necessary for ASE A9 certification. Taking a â&#x20AC;&#x153;strategy-based diagnosticâ&#x20AC;? approach, it covers how to maintain, diagnose, and repair light and mediumduty diesel engines, increasingly common in North American, Asian and European vehicles and trucks. n Fully explains contemporary clean diesel technology, from basic combustion principles to engine design and operation to advanced topics in electronic engine controls, injection and emission systems n Detailed coverage of the latest diesel engine technologies, such as exhaust aftertreatment systems, gas sensor technology, recirculation systems, common rail injection systems, and on-board diagnostic systems n Focus on technology used in the newest engine models from Ford, FCA, GM, and Mercedes-Benz n Emphasis on understanding emission systems operation and adhering to industry standards during the service and repair of diesel engines n Forecasts future directions of diesel engines in the context of the VW scandal and new GHG emission standards n Offers instructors an intuitive, methodical course structure and helpful support tools n Outcome focused with clear objectives, assessments, and seamless coordination with tasksheets Ideal for courses in light and medium duty diesel engines, engine management, or fuel systems Light Vehicle Diesel Engines equips students with the skills to successfully maintain and repair a new generation of diesel engines. About CDX Master Automotive Technician Series Organized around the principles of outcome-based education, CDX Learning Systems prepares students for workplace success by aligning training content and assessments to required professional competencies. The CDX Master Automotive Technician Series will eventually cover all areas of ASE certification. Visit www.cdxlearning.com to learn more.